Applications of Pulsed Laser Ablation in Li-Ion Battery Research

Abstract

Harnessing pulsed laser ablation processes in the manufacturing of energy storage devices is a new and promising strategy for the facile development of next-generation Li-ion batteries. In laser ablation, a pulsed laser is focused on a material surface such that the transfer of energy causes the removal of localized material via high throughput and environmentally-friendly processing. This chapter will provide a summary of the recent advances in laser ablation technologies for producing Li-ion battery materials and components. In terms of electrode optimization, it will examine the use of pulsed lasers to: (1) generate large specific surface area nanoparticles of active materials or stable integrative anodes; (2) deposit compositionally complex and stoichiometric thin film active materials; (3) create electrode architectures with increased Li-ion diffusion kinetics, enhanced wettability or free space to accommodate Si anode volume expansions, and; (4) remove the superficial inactive or solid electrolyte interface layers from electrode surfaces. It will also investigate the laser ablation of current collectors to produce textures with improved adhesion and the use of pulsed lasers for cutting and structuring solid ceramic electrolyte. Finally, this chapter will discuss the application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for chemical composition analysis of Li-ion batteries throughout their operating cycle.

1. Introduction

Mounting concerns associated with global warming and environmental degradation have resulted in increased efforts towards electrification. In particular, ground transportation is responsible for 15% of worldwide CO₂ emissions [1] and therefore constitutes one of the most consequential candidates for electrification. The development of energy storage technologies is vital in accommodating the transition towards electrical propulsion. Due principally to their high specific energy densities, long cycle lives and high Coulombic efficiencies, lithium-ion (Li-ion) batteries are currently considered the most suitable energy storage candidates for electric vehicle (EV) applications [2]. As a result, significant resources have recently been placed over the last two decades on optimizing Li-ion battery technologies.

1.1 Li-ion batteries

Li-ion batteries rely on the concept of extremely low electrode potential and the tendency of a Li atom to lose its single valence electron to become a charged Li⁺ ion. Figure 1 provides a schematic of the Li-ion battery operating mechanism. A typical Li-ion battery consists of an Al-foil current collector connected to a Li transition-metal oxide cathode and a Cu-foil current collector connected to a graphite anode. The cathode is a supplier of Li⁺ ions, while the anode is a storage medium for Li⁺ ions. Electrolyte positioned between the cathode and anode allows for the flow of Li⁺ ions while inhibiting the passage of electrons. To prevent short-circuiting, an insulating separator is included, with pores for Li⁺ permeation. In the charge state, a Li-ion battery is charged by applying a voltage by a power source which attracts the metal oxide valence electrons to its positive terminal. These electrons flow from the metal oxide cathode, through the external circuit to reach the graphite anode of the battery. Simultaneously, the Li⁺ ions are attracted to the negative terminal of the power supply and flow through the electrolyte to reach the anode. A Li-ion battery is considered to be fully charged once all of the active surface sites of the anode are occupied by the Li⁺ ions. However, this is an unstable state. As a result, the battery can act as an energy source (i.e., a galvanic device) by applying a load which allows the electrons to flow once again through an external circuit to return to the cathode. To maintain neutrality, the Li⁺ ions are released from the anode and flow through the electrolyte and separator to the cathode. Li⁺ intercalation facilitates charging and discharging of Li-ion batteries. In the charge state, the cathode is oxidized and delithiated while the anode is lithiated. Similarly, in the discharge state, the anode is delithiated while the cathode is reduced and lithiated.

Figure 1.

Schematic of a Li-ion battery operating in the (a) charge and (b) discharge states.

Li-ion battery research over the last two decades has looked to optimize all battery components through materials selection and modification in order to achieve improved performance, cost and operational safety. Since energy density is limited by the cathode (when conventional graphite anodes are used), the optimization of cathode materials has received significant attention. In particular, Co-, Mn-, and Ni-based materials have been investigated for their high electrochemical potentials which permit large battery cell voltages [2]. Polyanionic PO₄⁻ based cathodes have also been explored since they are significantly less susceptible to thermal runaway [3]. Even organic photoactive cathodes with improved specific capacity under illumination have been reported [4]. Evidently, cathode optimization research is ongoing as researchers look to further improve ionic and electronic conductivity, inhibit the formation of passivation layers, reduce costs and support operational safety [5]. In terms of anode optimization, Si- and Sn-materials with higher specific capacities than graphite have been proposed. In particular, Si-based anodes enable the highest theoretical capacity—4200 mAh/g—approximately 10 times higher than that of graphitic carbon anodes. However, severe volume expansion under lithiation limits their practical application due to structural compromise which results in poor electrical contact with the current collector [5]. Li-metal has similarly been investigated as an anode material for its high theoretical capacity and low electrochemical potential, however, dendrite formation causing short circuiting threatens device safety [2]. Solid-state electrolyte must therefore be employed with Li-metal anodes, but low ionic conductivity, as well as rigidity and roughness yield compromised contact resistance. Table 1 summarizes the key advantages and disadvantages of various Li-ion battery anode active materials. In terms of current collectors, efforts are centered around reducing foil thickness in order to reduce the total weight of devices. However, thin current collectors sacrifice electrical conductivity and suffer from poor mechanical integrity, which yields increased contact resistance with cycling [6]. Therefore, it is clear that while Li-ion batteries have already been deployed commercially, they stand nevertheless to gain from further research and development.

Anode active material	Key advantages	Key disadvantages
Graphite	Commercial Li-ion battery anode active material due to very long cycle life and good energy density	Energy density is not as high as alternative materials
Si	Very high specific capacity	Poor electronic conductivity and volume expansion under lithiation limits cycle life
Sn	Very high specific capacity	Volume expansion under lithiation limits cycle life
Li metal	Very high specific capacity and low reduction potential	Solid state electrolyte required to prevent short-circuiting

Table 1.

Comparison of the key advantages and disadvantages of various Li-ion battery anode active materials.

1.2 Pulsed laser ablation

Pulsed laser ablation is a commercially-relevant method for synthesizing and modifying materials and components in next-generation Li-ion batteries. In pulsed laser ablation, a laser beam is focused on a material or component surface. Absorption of the laser energy results in the sublimation and removal of superficial material. Ultra-fast pulse durations limit heat conduction, leaving the surrounding material in pristine condition, without physical or chemical alteration. At high flux, the laser will excite and ionize the plume of ejected material, forming plasma and inciting optical emissions [7].

Laser ablation can be harnessed in two modes, where the desired product is either: (1) the ejected material, or; (2) the ablated solid surface. Ejected material is the desired product in the generation of nanoparticles and thin films by pulsed laser ablation. In each case, the laser parameters are tuned to eject material with specific morphologies. Conversely, machining is the most common application of pulsed laser ablation where the desired product is the ablated solid surface. Importantly, laser ablation permits high throughput and environmentally-friendly processing of materials and can easily be tuned by manipulating the pulse duration, pulse frequency, wavelength, energy density and scanning speed of the laser.

In Li-ion batteries, pulsed laser ablation has been employed for synthesis, modification and analysis of materials and components via: (1) nanoparticle generation; (2) thin film deposition; (3) machining, and; (4) chemical composition analysis. As depicted in Figure 2, this chapter will examine the applications of pulsed laser ablation in next-generation Li-ion battery research.

Figure 2.

Outline of pulsed laser application areas in Li-ion battery research.

2. Pulsed lasers for nanoparticle generation

Pulsed lasers provide a robust and facile means of generating nanoparticles to be used as large surface area active materials in Li-ion battery anodes and cathodes. In general, the push towards smaller-sized materials comes as a result of the Li⁺ insertion/extraction processes at the anode and cathode, which are limited by the rate of Li⁺ diffusion. By incorporating nano-sized active materials at the cathode and anode, shorter lengths are obtained for both electronic and ionic transport. Compared to the conventional methods, in the laser-based process, no chemical additives or reagents are required (although surfactants can be added to prevent agglomeration), and there are no reaction by-products. Nanoparticles can either be generated by a two-step pulsed laser ablation in liquid (PLAL) process or a single-step in-situ laser ablation oxidation process.

2.1 Pulsed laser ablation in liquid for nanoparticulate active materials

In PLAL, the laser beam is focused on a solid target material submerged in liquid media and the ablated material is dispersed into the liquid [8]. Figure 3 provides a schematic of the PLAL process. Importantly, PLAL permits for recovery of the ejected material in nanoparticulate powder form. In other words, PLAL is distinguished in that the generated nanoparticles remain detached from either the target material or a substrate due to their suspension in liquid media. In order to recover the nanoparticles, a second separation step, involving drying or centrifugation, is required.

Figure 3.

Schematic of the pulsed laser ablation in liquid (PLAL) process.

In 2007, Tsuji et al. employed PLAL to generate LiMn₂O₄ nanoparticulate active material for improved cathode performance [9]. In particular, they hypothesized that the discharge capacity would improve in the large surface area nanoparticulate active material samples as a result of a decrease in Li diffusion distance. Tsuji et al. irradiated a suspension of LiMn₂O₄ powder in water with a Nd:YAG laser at 150 mJ/pulse for 1 hour and observed a 90% reduction in particle size from 5 μm to <100 nm. However, they did not observe any improvements in discharge capacity for current densities above 10 mA/cm² and reported a 25% reduction in discharge capacity for current densities below 10 mA/cm². The unexpected result is explained by the degradation of LiMn₂O₄ to MnO and Li, and Li/Mn cation mixing under laser ablation. Nonetheless, this work is significant in that it constitutes one of the first of its kind to employ PLAL for nanoparticulate active materials in Li-ion batteries and serves as a starting point to demonstrate the potential and opportunities for improvement of this method.

In 2015, Nowak et al. utilized PLAL to optimize Li-ion battery anodes via doping with SnO nanoparticles to increase the specific capacity [11]. The SnO was synthesized by focusing a Nd:YAG laser for 45 minutes on a metallic Sn target submerged in water. An energy density of 20 J/cm², a repetition rate of 10 Hz, and a pulse duration of 6 ns were used as ablation parameters. Subsequently, the SnO nanoparticles were embedded in a carbon matrix by mixing with gelatine and pyrolyzing at 900°C to form the SnO@C_gel anodic active material. The final electrode incorporated 60% active material, 20% carbon black and 20% binder, by mass. Importantly, the selected active material demonstrated improved performance as a result of: (1) the PLAL-generated SnO nanoparticles, which enhanced capacity, and; (2) the organic matrix, which provided stable cyclability. After 140 cycles at a current density of 100 mA/g, Nowak et al. reported a capacity of 580 mA h/g for their Li-ion battery, corresponding to a 98% retention in capacity.

Given the importance of Li-ion batteries in supporting transportation electrification, it has become crucial to devise methods of extending cycle life. The formation of a solid electrolyte interface (SEI) passivation layer on the anode, which restricts Li⁺ insertion, is to blame for compromised long-term capacity stability. To overcome this issue, Li₄Ti₅O₁₂ (LTO) has been investigated as an alternative to conventional graphite anodes for its high working potential which can suppress SEI layer formation. However, LTO suffers from small Li⁺ diffusion coefficients, limiting capacity. Recently, Alrefaee et al. investigated the use of PLAL to reduce LTO particle size and improve Li⁺ diffusion [10]. They submerged a solid LTO target in liquid polyethersulfone (PES) and ablated it with a Nd:YAG laser at a repetition rate of 10 Hz and a pulse duration of 7 ns for 60 min. As shown in Figure 4(a)–(c), by changing the energy from 40 mJ/pulse to 120 mJ/pulse, Alrefaee et al. were able to reduce the size of LTO nanoparticles from 26 nm to 5 nm. Electrochemical testing demonstrated superior performance with reduced particle size: in Figure 4(d), initial discharge capacities at 1 C were 180 and 244 mA h/g for LTO particle sizes of 26 and 5 nm, respectively. Similarly, in Figure 4(e), after cycling at high rates, discharge capacities remained the highest for the smallest LTO particle size.

Figure 4.

TEM images of LTO nanoparticles generated with laser energy of: (a) 40, (b) 80, and (c) 120 mJ/pulse, (d) cycle performance at 1C and (e) rate capability of electrochemical cells with different with LTO particle sizes of 26 nm (S1), 12 nm (S2) and 5 nm (S3). Reprinted with permission from [10]. Copyright (2023) Elsevier.

2.2 Laser ablation oxidation for metal oxide integrated anodes

In laser ablation oxidation, a nanoparticulate metal-oxide layer, XO_x, is grown on its pure metal precursor, X, to from an integrated XO_x-X material. As depicted in Figure 5, growth arises when: (1) the pulsed laser ablates a target metal surface, vapourizing and/or melting atoms under its high energy-intensity radiation; (2) the vapor/liquid-phase metallic atoms collide with oxygen atoms under atmospheric conditions, resulting in a loss of kinetic energy, atomic clustering, solidification and oxidation, and (3) the solid metal-oxide clusters are re-deposited onto the target metal surface. Laser parameters such as the frequency, scanning speed, pulse energy and pulse width can be tuned to control the size of metal oxide nanoparticles. Importantly, the use of lasers for integrated nanoparticulate metal-oxide growth is economical due to its simplicity and scalability. Additionally, the growth mechanism, which relies predominantly on the photothermal transfer of laser radiation to vapourize a metal target, is robust and widely applicable across metallic materials.

Figure 5.

Schematic of the laser ablation oxidation process for fabricating an XO_x-X integrated electrode.

The principles of laser ablation oxidation were first applied in Li-ion battery research in 2015 by Zhong’s group from Tsinghua University, as a means of accommodating the volume expansion plaguing stability in high-capacity Si-based anodes. The porous structures in silicon-based nanomaterials provide empty space for volume expansion under lithiation, reducing strain and improving cyclability. However, the conventional wet chemical- or chemical vapor deposition-based synthesis techniques do not permit the simultaneous achievement of high-throughput conditions, while ensuring precise microstructural control. In response, Zhong et al. introduced the laser ablation oxidation of monocrystalline Si wafers to form SiO_x-Si integrated anodic active material for Li-ion batteries, using a pulsed laser ($\lambda$ = 532 nm) with a frequency of 1000 Hz, a pulse width of 10 ns and an energy of 1.5E-4 J [12]. Using this novel method, their porous SiO_x-Si anodes achieved a high initial discharge capacity of 1400 mAh/g, and retained a capacity of 960 mAh/g after 800 cycles (more than double the capacity attained after 800 cycles with commercial graphite anodes). Similar work has since been replicated using an IR laser [13].

Building on their previous work, Zhong et al. discovered a new application for laser ablation oxidation in Li-ion batteries: the in-situ growth of metal oxide active materials on metallic current collectors [14]. This evolution of the laser ablation technology takes advantage of the existing porous metal oxide nanostructures to prevent strain upon volume expansion, but also renders unnecessary the use of binder, thereby improving the adhesion of active material to the current collector and the electronic conduction of the anode. They used the same 532 nm laser with a frequency of 30 kHz, a pulse width of 12 ns and a pulse energy of 0.4 mJ to ablate Cu foil, forming a CuO-Cu integrated anode. Remarkable stability was achieved: the capacity after 800 cycles at 1.5 A/g was 394 mAh/g, corresponding to a Coulombic efficiency greater than 99%. In subsequent publications, Zhong et al. demonstrated the robustness of the laser ablation oxidation technique, reporting the preparation of binder-free CoO-Co, NiO@C-Ni, Fe₂O₃/Fe₃O₄@C-Fe, and MoO₃-Mo integrated anodes [15, 16].

3. Pulsed lasers for thin film deposition

Pulsed lasers can be used to grow 2D materials in a widely employed and mature technique known as pulsed laser deposition (PLD). As shown in Figure 6, PLD is a vacuum-based deposition method, wherein a pulsed laser beam is focused on the surface of a solid target material. The laser photons are converted into thermal, chemical and mechanical energy, resulting in the rapid extraction of material from the surface of the target to form a plasma plume which condenses on the substrate [17].

Figure 6.

Schematic of a pulsed laser deposition system.

The PLD technique can be used to produce high-quality and dense films with tunable morphologies. However, since large-scale deposition is challenging, PLD may be particularly well-suited to the manufacturing of thin film batteries. Given the prevalence of microelectronics in the technology market, there is a heightened need for small, lightweight batteries, offering long lifetimes and high energy densities. Solid-state thin film batteries are a proposed solution, wherein all battery components are thin films that amount to a multilayer device. Physical vapor deposition techniques such as PLD are attractive in the fabrication of thin film batteries because they negate the need for binder, thereby improving energy density.

The PLD technique yields highly uniform films with preserved stoichiometry; it is ideally suited for producing novel thin film compositions and structures. The fabrication of compositionally complex thin films containing lithium is particularly challenging because lithium is a very easily evaporated element, rendering stoichiometric preservation difficult by other deposition methods. As a result, PLD is considered one of the best techniques for producing thin film cathode active materials.

LiCoO₂ is considered one of the best cathode active materials because of its high energy density and superior cycling stability. As such it is already being employed in commercial Li-ion secondary batteries. Antaya et al. published one of the first instances of using the PLD technique under optimized processing conditions to produce nominally stoichiometric LiCoO₂ thin films with near-zero Li loss [18]. They employed a pulsed XeCl laser ($\lambda$ = 308 nm) with a fluence of 5 to 6 J/cm² to ablate a solid LiCoO₂ target under oxygen atmosphere. A post-deposition annealing step at temperatures up to 700°C was applied to produce crystalline films. The authors studied the effects of crystallinity and on the electrochemical properties and found that the thin films of LiCoO₂ annealed above 500°C had the largest fractions of high-temperature LiCoO₂ crystalline phases and correspondingly exhibited the highest capacities.

Since PLD permits the facile deposition of compositionally complex thin films, researchers have used it to further tune the chemical composition of conventional LiCoO₂ active materials. In order to increase battery voltage and reduce toxicity, Perkins et al. substituted half of the Co in LiCoO₂ for Al, producing thin films of LiCo_0.5Al_0.5O₂ [19]. Al substitutions are also economical in that Al is lighter and less expensive than Co. They employed the PLD technique to ablate stoichiometric ceramic targets of LiCo_0.5Al_0.5O₂ using an excimer laser ($\lambda$ = 248 nm) at 10 Hz and 325–415 mJ/pulse. The authors found that LiCo_0.5Al_0.5O₂ cathodes exhibit higher voltages during charging cycles than LiCoO₂ cathodes, however they report significant hysteresis between charging and discharging cycles, ultimately resulting in a 200% reduction in capacity for the LiCo_0.5Al_0.5O₂ cathodes.

While PLD typically yields thin films that are stoichiometrically equivalent to the target material, due to the volatility of the atoms involved, Li and O deficiencies are not uncommon. This is particularly true in the deposition of LiMn₂O₄, which is of interest in micro-batteries for its safer overcharge tolerance. In order to overcome compositional deficiencies, the PLD parameters such as the deposition temperature, oxygen pressure and substrate-to-target distance should be tuned. For instance, in order to prevent Li deficiencies that occur as a result of Li-out diffusion, films should be grown at substrate temperatures less than 400–600°C [20, 21]. Moreover, Li deficiency is increased when the distance between the substrate and the target is large [22]. Interactions between laser-produced plasma and oxygen background gas are not well-understood. However, it is generally agreed upon that Li and O deficiencies emerge in films grown under high vacuum or low (<10 Pa) oxygen pressures [20, 23]. This may be related to a slowing of the laser-produced plasma ions with increasing background gas pressures and a preferential re-sputtering of Li under vacuum or low background gas conditions [20].

PLD has also been used to optimize Li-ion battery cathodes via the addition of functional thin film surface layers. As a result of electron exchange during operation, a solid electrolyte interface (SEI) layer forms at the LiCoO₂ cathode-electrolyte interface, increasing the resistance to further charge transfer and limiting capacity retention. Teranishi et al. used the PLD technique to cover their LiCoO₂ active material with BaTiO₃, thereby suppressing the formation of native SEI [24]. They were able to achieve three-dimensional LiCoO₂ coverage by modifying the conventional PLD configuration: as shown in Figure 7(b), the plasma plume was directed into a dynamically-mixed pan containing LiCoO₂ powder, such that BaTiO₃ nanoparticles were deposited onto the LiCoO₂ particle surfaces. The dielectric BaTiO₃ was multifunctional in that it not only prevented SEI formation, but also provided pathways with minimized activation energy for Li migration. Electrochemical performance testing, given in Figure 7(c), demonstrated both improved rate capability and capacity retention with BaTiO₃ incorporation by PLD. Notably, after cycling at a high current rate of 10 C for 50 cycles, the PLD BaTiO₃ cathodes retained $~$80% capacity, compared to only 25% for those without BaTiO₃ decoration.

Figure 7.

Schematic representation of (a) the conventional PLD method, and (b) the PLD-based nanodecoration of BaTiO₃ on LCO powder, (c) Cycle performance at 10 C of BaTiO₃ nanodecorated LCO after different numbers of laser ablation pulses (N_Lp) compared to bare LCO. Reprinted with permission from [24]. Copyright (2022) AIP Publishing.

While the compositionally complex nature of lithium transition metal oxide cathode active materials makes them ideal candidates for fabrication via the PLD technique, all thin film battery components can be grown by PLD. As a result, the PLD of anode materials [25, 26, 27, 28] and solid electrolyte [29, 30, 31] has also been investigated extensively. In fact, to achieve the most economical thin film batteries, all battery layers should be deposited by a single technique, without breaking the vacuum. In this pursuit, Kuwata et al. were the first to report the fabrication of a complete thin film Li-ion battery using only the PLD technique [32]. They employed a Nd:YAG laser ($\lambda$ = 266 nm) at a frequency of 10 Hz and a fluence of 3.5 J/cm² to sequentially ablate various target materials under oxygenated environment. Following this methodology, the authors successfully demonstrated the production of an all-PLD thin film battery consisting of a: (1) Pt/Cr cathode current collector; (2) LiCoO₂ cathode active material; (3) Li₂O-V₂O₅-SiO₂ (LVSO) solid electrolyte; (4) SnO anode active material, and (5) Pt anode current collector.

4. Pulsed lasers for machining

Instead of adding functional material layers or nanoparticles, pulsed lasers can equally be used in the removal of material to create textures or microstructures. Also known as laser micro-milling or laser micromachining, the use of pulsed lasers to accurately machine fine features and complex geometries into a material surface is important in industries and applications requiring a degree of precision that is not achievable by conventional mechanical machining [33]. In laser micromachining, a pulsed laser with specific frequency, fluence and beam radius is scanned across a material surface. Ultra-fast pulse frequencies ensure that photons are absorbed faster than heat can be diffused out of the irradiated region, resulting in localized ablation without any damage to the surrounding area. For this reason, low-melting temperature materials such as plastics can be laser micromachined. Fluence can be increased to achieve higher rates of ablation and deeper textures, however excess fluence can compromise quality by heating the material surface. Finally, the radius of the laser beam will directly determine the machining resolution; by manipulating the focusing optics, machining resolutions from hundreds of nanometers to hundreds of microns can be achieved. The type of laser must also be carefully selecting according to the application. In particular, the use of UV lasers is common because UV light is readily absorbed by most materials. Importantly, laser micromachining is positioned as a green manufacturing process: since the heat-affected zone produced during laser micromachining is limited, energy is used efficiently and consumption is minimized, the need for cooling water is eliminated, and post-processing steps such as polishing are rendered unnecessary.

4.1 Creation of 3D architectures

The microstructuring of electrode materials to increase electrode surface area and enhance Li-ion diffusion kinetics is considered an important avenue for next-generation Li-ion battery development. An increasingly electric society has called for batteries with improved energy density. The use of thick electrodes with an increased ratio of active material to inactive current collector and separator foils has been proposed as a straightforward solution. However, there exists an inverse relationship between specific energy and specific power, such that thick electrode batteries often exhibit increased resistance to diffusion, compromising rate capability and long term cyclability. In order to overcome the challenges associated with thick electrodes, pulsed laser micromachining has been employed to generate 3D active material architectures with enhanced diffusion kinetics. A schematic of the electrode laser-structuring process is given in Figure 8(a).

Figure 8.

(a) Schematic of the electrode laser-structuring process. (b) SEM image of laser-structured NMC622 cathodes (91 μm of active material on Al current collector) with a 200 μm pitch distance. (c) Ragone plots for unstructured and laser-structured cells with active material thicknesses of 91, 151 and 250 μm. Reproduced with permission from [34]. Copyright (2019) MDPI.

This has been demonstrated for many cathode active materials, including LiMn₂O₄ [35, 36], LiFePO₄ [37] and Li(NiMnCo)O₂ (NMC) [34, 38, 39]. For instance, Pröll et al. used a fiber laser at λ = 515 nm with a pulse energy and width of 0.125 μJ and 350 fs, respectively, to create 3D grid architectures with an aspect ratio of $~$1.7 in 60 μm thick LiMn₂O₄ cathode active material [35]. They also employed calendering to improve electrical contact with the current collector. Galvanostatic and cyclic voltammetric testing confirmed the improved diffusion kinetics of the laser-structured electrodes, ultimately resulting in a 50% increase in capacity retention at 1C compared to its untreated counterpart.

Zhu et al. employed a similar approach for simultaneous optimization of energy and power density in Li-ion batteries with Li(Ni_0.6Mn_0.2Co_0.2)O₂ (NMC 622) cathodes [34]. Line-patterning NMC622 cathodes with a λ = 1030 nm laser (SEM image given in Figure 8(b)) revealed the facile tuning of battery performance for various applications: (1) thick (>200 μm), non-laser-textured cathodes provide the largest energy density; (2) thin (<100 μm) laser-structured cathodes provide the highest power, and; (3) thick, laser-structured cathodes provide both high power and high energy density. Ragone plots demonstrating these findings are given in Figure 8(c).

Laser surface modification technologies have also garnered significant attention across biomedical, oil, automotive, etc. industries for their ability to change surface wetting characteristics. Compared to conventional chemical-based techniques, tailoring wettability via laser surface modification is more environmentally friendly and long-lasting [40]. Surface wettability is of particular interest in Li-ion batteries because Li-ion diffusion only occurs across wetted electrode surfaces. Moreover, electrolyte filling due to poor surface wettability is a major bottleneck in the manufacturing of batteries. As a result, laser surface modification technologies have been harnessed to increase electrode wettability. Kleefoot et al. used a low-energy nanosecond pulsed laser to selectively thermally decompose the amorphous binder and carbon black phases in graphite active materials, thereby increasing the surface concentration of crystalline graphite particles [41]. The result is a roughened surface without a loss of active material. Importantly, by roughening the surface of graphite active materials via the aforementioned methodology, Kleefoot et al. reported a pronounced 8-fold increase in wetted surface area.

Finally, pulsed lasers can be used to create electrode architectures with free space or porosity to accommodate Si anode volume expansions, thereby advancing silicon-based anode implementation. Zheng et al. demonstrated that mechanical stress in silicon/graphite composite electrodes is significantly reduced as a result of laser-generated electrode structuring, leading to improved capacity retention, cycle stability and cell lifetimes [42].

While significant improvements in battery performance have been demonstrated with laser-texturing of electrodes, the question of commercial viability remains. In particular, in order to avoid bottlenecks in manufacturing, laser patterning speeds of several m/min must be demonstrated. With the objective of scaling the laser ablation technologies for commercial battery manufacturing, Meyer et al. studied the impact of laser processing parameters on resulting graphite and silicon/graphite electrode architectures [43]. As demonstrated in Figure 9, they observed an important trade-off between laser scanning speed and aspect ratio. Nonetheless, their investigation revealed that laser power can be increased to compensate for the reduction in aspect ratio. Notably, Meyer et al. were able to demonstrate high aspect ratio ($~$1.8) electrode texturing through the active material layer and down to the current collector at laser powers of 180 W for fast scanning speeds up to 1.7 m/s (as indicated by the red arrow in Figure 9).

Figure 9.

Aspect ratio of the grooves in laser-microstructured silicon/graphite composite electrodes as a function of laser scanning velocity or repetition rate for laser powers from 12 to 240 W. Solid symbols indicate that the active material was textured through to the current collector, while open symbols indicate that texturing did not reach the current collector. Reproduced with permission from [43]. Copyright (2023) Laser Institute of America.

In an effort to improve data accessibility and accelerate commercialization, Tancin et al. have recently published a thorough comparative study of the effect of laser ablation parameters on the resulting ablated microstructures for several electrode materials including graphite, silicon, NMC and LTO [44]. Importantly, the authors examined industrially-relevant ablation metrics such as repeatability, ablation rate and aspect ratio.

4.2 Regeneration via surface layer removal

In place of partial surface layer removal—as in the creation of 3D architectures—pulsed laser ablation can be similarly employed to remove the entire surface layer of a component. This form of ablation is appropriate when components are contaminated during operation or rendered inactive due to chemical reactions with their operating environment. In fact, lasers are commonly used in manufacturing industries to extend the service life of parts by removing mechanically or electrically compromised surfaces before failure occurs. Often termed “laser cleaning,” this process is common across semiconductor, welding and shipbuilding industries to remove oxide layers such as rust and contaminants such as millscale, salt and oil [45].

Currently, commercial Li-ion batteries are plagued by capacities that decrease over their lifetimes. For instance, in EV batteries, capacities degrade by an average of 2.3% annually [46]. Since these batteries are considered to have reached end-of-life by the time they degrade to 70–80% of their original capacity [47], lifetimes of less than 9 years are to be expected. This degradation is predominantly a result of the formation of solid electrolyte interface (SEI) layers on battery anodes, which increase resistance and trap lithium ions, reducing the total amount of lithium available for intercalation. The SEI is composed of oxidation and reduction reaction products, namely Li₂CO₃ [48] and LiF [49], depending on the electrolyte used. Chemical methods have been shown to effectively remove SEI and restore capacity [50], however their toxicity limits adoption. Pulsed laser ablation therefore emerges as a sustainable alternative for Li-ion battery regeneration. Moreover, the small heat affected zones generated under ablation with ultrafast pulsed lasers prevents energy dissipation and damage to subsurface layers.

Zhang et al. demonstrated the feasibility of using pulsed laser ablation for SEI layer removal [51]. They utilized a Nd:YAG laser in the third harmonic with λ = 337 nm at an energy density of 55 mJ/cm² and a pulse width of 7 ns. Successful removal of the SEI without damaging the graphite active material was confirmed by SEM and Raman. Figure 10(a) and (b) provide a comparison of the graphite anode before and after 70 galvanostatic cycles, respectively. Before cycling, smooth graphite surfaces and sharp edges are apparent. Comparatively, the SEI forms after cycling, making the anode surface appear dull and coarse. With laser treatment, Zhang et al. were able to restore the smooth and sharp features of the graphite (Figure 10(c)), indicating the removal of the SEI layer. The Raman spectra given in Figure 10(e) confirm the regeneration of the graphite anode via similar I_D/I_G ratios (i.e., no change in graphite structural defects) for the pristine and laser-treated samples.

Figure 10.

SEM images of the graphite anode taken (a) before and (b) after 70 galvanostatic cycles. After cycling, the electrode was laser-treated to remove the SEI layer. SEM images show the (c) ablated and (d) unablated regions. (e) Raman spectra of the graphite electrode in the pristine condition (before cycling), compared to after cycling and laser-treatment. Reproduced with permission from [51]. Copyright (2017) Laser Institute of America.

4.3 Cutting solid electrolyte

Pulsed lasers are similarly being investigated for their role in processing solid ceramic electrolyte in the scaled and economic production of all-solid-state batteries. Solid ceramic electrolyte layers typically have thicknesses less than 100 μm, and they must remain defect-free after processing in order to prevent short circuiting between the cathode and anode [52]. Melt-quenching [53] and sintering [54] are the conventional methods used to produce low-porosity solid ceramic electrolyte, however, these often cause distortion and changes in geometry that have downstream consequences such as compromised contact with the adjacent electrodes. By applying machining to cut the solid electrolyte layers post-densification (i.e., remove distorted regions), precise geometries with improved mechanical properties can be restored. Laser ablation emerges as the most suitable machining method for processing solid ceramic electrolyte due to the hardness, brittleness and thin nature of the solid ceramic electrolyte. As a result, recent work has demonstrated the feasibility of using nano and picosecond lasers for cutting thin sheets of lithium aluminum titanium phosphate (Li_1+xAl_xTi_2−x(PO₄)₃, LATP) [55] and lithium aluminum germanium titanium phosphate (Li_1+x+3zAl_x(Ti,Ge)_2−xSi_3zP_3−zO₁₂, LAGTP) [56] solid electrolytes.

5. Pulsed lasers for chemical composition analysis

A final application of pulsed lasers in Li-ion battery research is the use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for chemical composition analysis of Li-ion batteries throughout their operating cycle. This technique is positioned as means of understanding the underlying mechanisms related to SEI formation and can help to inform optimization strategies. Other analysis techniques such as X-ray photoelectron spectroscopy (XPS) [57], time of flight-secondary ion mass spectrometry (Tof-SIMS) [58] and electron energy loss spectroscopy (EELS) [59] have been used to investigate the composition of SEI on Li-ion battery anodes. While these techniques provide insight into composition at depths resolved to the range of a few nanometers, they have limited lateral resolution. Conversely, the LA-ICP-MS technique provides a lateral spot resolution on the order of several tens of μm, enabling the facile analysis of entire electrode surfaces via rapid scanning.

As depicted in Figure 11, in LA-ICP-MS, a pulsed laser is used to ablate the surface of a sample. The particles generated by ablation are transported via argon carrier gas to an ICP plasma torch, where they are ionized. The ionized particles are then transferred to a mass spectrometer for elemental and isotopic analysis [60]. Currently, the most common applications of LA-ICP-MS are in geological research [61].

Figure 11.

Schematic of LA-ICP-MS. Reproduced with permission from [60]. Copyright (2017) De Gruyter.

In 2017, Schwieters et al. first demonstrated proof-of-concept for the use of LA-ICP-MS to quantify the amount of lithium in SEI layers on aged graphite anodes [62]. Since lithium immobilized in the SEI contributes to decreased capacity, this work helps to advance the study and quantification of Li-ion battery health.

Another degradative phenomenon occurring during cyclic aging of Li-ion batteries with transition metal oxide cathodes is the dissolution of transition metal cathode elements like Ni, Mn and Co and their re-deposition on the anode. Schwieters et al. applied LA-ICP-MS to investigate the influence of operating parameters on transition metal deposition [63]. They found that cells operating at higher voltages developed significantly more lithium and transition metal deposits, indicated increased SEI formation. This work demonstrates the promise of using LA-ICP-MS for compositional SEI analysis.

6. Conclusions

Pulsed laser ablation processes are scalable, environmentally sustainable and provide a high degree of machining precision. Moreover, machining in ultrafast pulses prevents thermal diffusion and permits robust material applicability. Consequently, pulsed lasers have been applied across many areas of Li-ion battery research, resulting in significant advancements. This chapter reviewed the use of pulsed lasers for nanoparticle generation, thin film deposition, machining and chemical composition analysis. In summary:

• Pulsed laser ablation in liquid was used to create nanoparticulate active material with high surface area, while laser ablation oxidation permitted integrated synthesis of nanoparticles directly on the current collector.

• Pulsed laser deposition was used to grow compositionally complex 2D thin films of cathode active materials in solid state microbatteries.

• Pulsed laser ablation was used to create 3D electrode architectures with improved diffusion kinetics and electrolyte wettability. Where Si-based anodes were used, laser texturing was used to accommodate volume expansions and reduce mechanical stress leading to failure.

• Pulsed laser ablation was used to regenerate the capacity of aged Li-ion batteries by removing the SEI layer from the anode surface.

• Pulsed lasers were used in the scaled cutting and processing of solid ceramic electrolyte.

• Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to analyze SEI composition.

Overall, this chapter demonstrates the benefits realized by applying pulsed lasers for processing materials and components in Li-ion battery manufacturing and research. These methods should continue to be applied at the research-scale in order to develop next-generation batteries with improved performance. Additionally, future work should focus on scaling the developed ablation methods to ensure commercial viability in large-scale battery manufacturing processes.

Conflict of interest

Dongfang Yang is the Editor and Katherine Gibson is the Co-Editor of this IntechOpen book entitled “Pulsed Laser Processing of Materials.”