Carbon-LiFePO4 Cathodes for Lithium-Ion Batteries: History
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Contributor:
Brindha Ramasubramanian
,
Subramanian Sundarrajan
,
Vijila Chellappan
,
M. V. Reddy
,
Seeram Ramakrishna
,
Karim Zaghib

Li-ion batteries are in demand due to technological advancements in the electronics industry; thus, expanding the battery supply chain and improving its electrochemical performance is crucial. Carbon materials are used to increase the cyclic stability and specific capacity of cathode materials, which are essential to batteries. LiFePO4 (LFP) cathodes are generally safe and have a long cycle life. However, the common LFP cathode has a low inherent conductivity, and adding a carbon nanomaterial significantly influences how well it performs electrochemically.

  • LiFePO4
  • carbon
  • Li-ion battery
  • Li-ion diffusion
  • electrical conductivity

1. Introduction

The growth of renewable energy technologies and the electric vehicles market requires developing low-cost batteries with high energy density. With high operating voltage (~3.2 V to 3.4 V), and relatively high capacity, Li-ion batteries (LIBs) have been used popularly in electric vehicles. However, the energy density of LIBs is often limited by the low electrochemical capacity of cathode materials (<200 mAh g−1) and the poor rate performance of the electrodes. LiCoO2 was the first commercially used cathode material (i.e., used by SONY electronics) in LIBs; however, it causes environmental pollution during manufacturing, overcharging while being used, posing a possible safety concern, and is costly given the low storage capacity [1]. LiNiO2 has an affordable price and analogous crystal structure to LiCoO2; regrettably, it is challenging to manufacture and has limited temperature and rate capability. Although the spinel structure LiMn2O4 is reliable and easy to make, the Jahn–Teller distortion in the lattice often during the cyclic process causes it to deform, leading to fast degradation of capacity, particularly at high temperature and stress levels. Lithium iron phosphate (LiFePO4) cathodes are relatively safer to use and have longer cycle life than LiNi1−x−yMnxCoyO2 (NMC) and LiNi1−x−yCoxAlyO2 (NCA) cathodes [1][2][3]. Though NMC cathodes account for a higher market share than LFP, Companies like Tesla are switching to LFP cathodes due to their long cycle life and thermal stability, up to 270 °C [4].
Destenay discovered LiFePO4 (LFP) in triphylite mineral solid solutions, forming olivine and isomorphous structure with Li phosphates of divalent Mn and Fe. Remarkably, Prof Goodenough and his colleagues’ ground-breaking work demonstrated the possibility of extracting Li from LFP using an insertion method for the first time [5]. He foretold the stability of sulfides, and phosphates at high oxidation states and mentioned the higher potentials (>3.2 V Vs. Li/Li+) for transition metal oxides [6]. Besides these efforts, LFP exhibited a low intrinsic conductivity reducing its operating efficiency. Prof Michel Armand and his coworkers postulated that adding carbon materials to LFP cathodes would be able to significantly improve their performance, allowing them to achieve a practical, reversible capacity comparable to the theoretical value (~170 mAh g−1). LFP rechargeable batteries have high specific energy (90–170 Wh Kg−1) and high volumetric energy density (1200 kJ L−1), indicating that they can deliver the required energy for powering EVs and grid; it is also important to note that specific LFP-based batteries offer good cyclic performance (~1500 cycles) with nominal cell voltage (~3.2 Vs. Li/Li+) and in commercial scale (18,650 cells) offers the cyclic performance of 10,000 cycles at 0.5 C, reported by Preger et al. [7]. LFP is a benign substance that does not emit either harmful or toxic gases, safer for humans and the ecosystem. LFPs are resistant to chemicals (oxygen) and less combustible; therefore, they are less vulnerable to thermal runaway and withstand temperatures up to 270 °C. LFP cathodes are less expensive at ~353,000 won per kWh than NCM622 cathodes at ~385,000 won per kWh, [8] and Fe metal is abundant in the Earth’s crust (50,500 ppm, according to Israel Science and Technology) [9].
Though, LFP has undeniable benefits of its olivine structure, high theoretical capacity (~170 mAh g−1), safety, low cost, and environmental benignity [10], there exist a few disadvantages in LFP, namely, (i) low electron conduction (10−6 to 10−10 s cm−1), (ii) inadequate Li-ion diffusion, which limits their performance [11]. The broken octahedral sites of FeO6 in LiFePO4 diminish the speed of Li-ion diffusion across Fe–O–Fe (~10−14 cm2 s−1) [12]. Researchers improve the electrical conductance and ion migration in LFP by adding metal oxide, reducing LFP’s particle size, encapsulating a porous carbon layer, and multi-element doping [13]. Doping widens the Li+ migration pathway, enhances the output voltage, and working potential of LFP [12]. To develop LIBs with an operating voltage of 3.4 V and above vs. Li/Li+, the redox potential of the cathode materials must be boosted, which is considerably achieved by ion doping, being a trendy research hotspot [12]. Boosting the operating potential, on the other hand, is generally linked with a few constraints [2]. To avoid loss of the battery’s electrochemical efficiency, the potential must remain within the electrochemical stability window of the electrolyte (i.e., the battery can be recharged and discharged repeatedly without provoking thermal decomposition and redox failure of the electrolytes) [14]. Due to variations in free energy (∆G°) during the chemical reactions, the electrochemical stability window of the electrolyte becomes shorter than the energy barrier separating the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the electrolyte [12]. Furthermore, high output voltage could induce permanent structural changes in the cathode materials, compromising the electromechanical stability and decreasing the LIBs operational lifetime [12]. As a result, doping numerous ions necessitates a great deal of thought in terms of performance and post-doping outcomes. The Li+ migration route can be shortened by shrinking the size of LFP particles; however, this has the drawback of introducing the interface effect, where ions get crowded, resulting in uneven polarization and extensive electron generation; these disadvantages may cause thermal instabilities, necessitating optimization and management [14].
Coating of battery electrode materials has become essential for enhancing the performance, and protecting the surfaces from temperature, pressure, and stress [2]. Surface modification has proven to be a cost-effective and viable method for improving LFP’s performance by altering either the physiochemical properties or adding a protective barrier to reduce excessive interaction between the electrode layer and the electrolyte. Several multifunctional materials have already been employed to stabilize LFP, notably carbon, polymers, alloys, inorganic salts, nitrides, transition metal oxides, etc. Among them carbon nanomaterials improve the electrical characteristics of LFP crystals, are ubiquitous, and are simple to implement [14].
Carbon materials have received significant attention [15] because of the following benefits:
(i)
The large surface area, strong heat resistance, electrical conductivity, and high structural integrity of carbon promotes the performance of LIBs [16].
(ii)
Carbon compounds are available in various structures and dimensions, including graphene, graphite, reduced graphene oxide (rGO), carbon nanofibers, and nanotubes each with its own set of physical and chemical characteristics, allowing for a wide range of options for stabilizing electrodes [17].
(iii)
Sources for Carbon materials are plentiful and inexpensive [17].
(iv)
The light weight and compatibility of carbon compounds ameliorate the electrode space utilization in LIBs [18].
Carbon is one of the highly suitable materials to stabilise LFP due to the above-mentioned points, which contribute to their practical adaptability [16]. Adding carbon is one of the most effective ways to alter the properties of LFP, boosting the material’s electron transport and performance rate. Carbon addition influences the electrode’s electrochemical performance, determined mainly by fabrication quality, including graphitization level, sorption capacity, and spread on the electrode surface [17]. Although carbon treatment may significantly increase the conductivity of LFP, too much carbon may impede Li+ ion transport. A carbon matrix of 1 to 10 nm thick is generally adequate to ensure a stable passage of lithium-ions across the electrodes [19]. Further, different chemical origins and structures of carbon exhibit different characteristics and thereby impacting the electrochemical and interfacial interactions.

2. Effects of Adding Carbon to LFP Cathodes

2.1. Carbon Sources

Carbon materials offer high electrical conductivity, thermal stability, and chemical stability in various electrolytes, from severely acidic to basic. The electrochemical performance of the C/LFP electrodes is determined by the carbon precursor, such as organic, inorganic, polymer, and biomass.

2.1.1. MOFs-Derived Carbon

Metal organic frameworks (MOFs) are porous periodic structures created by organic ligands and metal ions. MOFs have recently been employed as precursors for in-situ synthesis of MOF-derived carbon cathode materials, which has sparked researchers’ interest and gained popularity. MOF-derived carbon exhibits high porosity, tunable pore size, high surface area, excellent cycle life, and charge storage capacity [20]; they are frequently produced by the hydrothermal or solvothermal method and used as templates to make carbon materials. As a conductive buffer layer, a porous carbon framework produced from MOFs by calcination and post-annealing treatments benefits the homogeneous electroactive species distribution. Dopants such as F, B, Cl, S, and O may also be added to MOF-derived carbon to improve electronic conductivity, active sites, and defect reduction [20]. Simultaneous doping of heteroatoms into carbon nanomaterials increases their electrochemical activity, resulting in a stable reversible capacity. 

2.1.2. Biomass and Organic-Derived Carbon

Biomass-derived carbon has eco-friendliness and renewability. After multiple refining, acid, and heat treatments, the biomass waste is transformed into porous carbon materials. Previously, pyrolysis, chemical activation, solid state synthesis and thermal decomposition processes were performed to synthesis carbon from biomass waste. Porous structures such as hierarchical pores, 3D pores, and cross-linked pores are reported for biomass-derived carbon [21]. The presence of surface functional groups is relatively higher in biomass-derived carbon than chemically-derived carbon; these additional functional groups promote the adsorption of ions, electrolyte penetration, and their reactivity and, at times, lead to the formation of different compounds during the electrochemical reactions. In addition, different sources of biomass have different chemical components. For example, carbon-rich biomass contains significant amounts of nitrogen, oxygen, and hydrogen. The natural existence of nitrogen, boron, and hydrogen in biomass, which persists as heteroatoms after converting into carbon, boosts porosity, electrical conductivity, and ion absorption; as a result, the overall electrochemical performance can be improved [21]. Other elements that may be found in trace levels include phosphorus, silica, calcium, magnesium, sodium, and potassium. Some commonly used biomass sources are rice husk, coconut shell, wheat bran, vegetable wastes, spinach, groundnut shell, and nut’s shell. Among them, coconut shells have popularly been investigated as promising carbon sources. One recent example is the use of coconut shell-derived rGO composited with LFP using mechanical ultracentrifugation technique, where the rGO and LFP were prepared separately using pyrolysis and sol-gel technique and mixed via ultrasonication. Initially, the rGO solution was added to LFP nanoparticles mixed with n-butanol and ultrasonicated till the formation of a slurry. Post heat treatments at 80 °C for several hours, the rGO/LFP cathodes were obtained [22]. The LFP particles adhere firmly on both sides of rGO layers with increased surface area and reduced Li-ion diffusion path; this post-synthesis mixing protocol opens up opportunities in the future to explore other biomasses and mixing procedures. The study also suggests that sp2 hybridised surface carbon on LFP performed electrochemically better than sp3 hybridised carbon [22].
Most recently, sericin, a protein layer that silkworms use in the creation of silk, was used as a bio precursor for preparing carbon-coated LFP. Sericin powder was obtained from silk worms by degumming process, mixed with LFP, and heat treated at 500 °C for 3 h in Argon atmosphere to obtain nitrogen-doped carbon coated LFP (N-C@LFP). The sericin layer is rich in nitrogen and carbon, thus creating nitrogen-based functional groups on the surface of C-LFP. The excellent discharge capacity of 113.5 mAh g−1 with 1 wt% of N-C@LFP was observed, which was 20% higher than bare LFP. 

2.1.3. Polymer-Derived Carbon

Polymers-derived carbon (PDC) has different surface functional groups depending on the base monomer. The conversion of polymers to carbon is not always perfect. Short-chain polymers break down rapidly in organic electrolytes, limiting battery cycle life. Long-chain polymers are stable and have negligible solubility in electrolytes. Polymers such as polypyrrole, polythiophene, PEDOT, polyvinyl pyrrolidone, and polyaniline are used to build porous carbon. Conducting polymers are more suited for obtaining carbon for its use in battery applications. In particular, highly graphitized carbons are produced when functionalized heterocyclic or ring-forming compounds are used in pyrolysis. Nien et al. compared the carbonization behavior of four different polymers, namely poly(ethylene oxide) (PEO), polybutadiene (PB), polystyrene (PS), and styrene-butadiene-styrene (SBS). The pyrolysis process was employed, and vinyl compounds (PS) were observed to form scissions. Main chain scissions develop without the formation of volatile chemicals during the pyrolysis of PS, and the carbonization process of unsaturated bonds and aromatic rings promotes the formation of graphite. The authors revealed that PS is the best suitable polymer for developing a fine particle size LiFePO4/C composite and an evenly coated carbon conductive layer with improved electrochemical performance. The main advantages of using PS as a polymer additive include coordinated LiFePO4 crystallization and PS pyrolysis [28].

2.2. Dimensions at Nanoscale

2.2.1. One Dimensional (1D) C-LFP

Carbon nanotubes (CNTs), a one dimensional carbon nanomaterial, with diameters ranging from 0.6 to 50 nm, commonly formed from a thin sheet of pure graphite, have excellent features of strength, thermal, and electrical conductivity that have been employed in the field of energy storage for the past two decades. The electrons navigate easily in 1D carbon materials. The increased surface area of multiple carbon coating, sandwich, mesoporous, and hollow structures leads to improved ion mobility. 

Effects of Synthesis, Particle Size, Diffusion Path and Bonding on 1D C-LFP

Firstly, the limited conductivity of LFP causes strong polarisation under significant charging rate conditions, causing a drop in overall capacity. In order to overcome this main issue and for making high-quality LFP cathodes, researchers used a variety of synthesis procedures [23]. Hydrothermal synthesis of LFP with carbon coating through heat treatment has been rejuvenated for fabricating the cathode in Li-based batteries because of its low cost, simplicity, and familiarity. One of the recent examples is the work done by Kanagaraj et al., which discusses the preparation of multiwall carbon nanotubes (MWCNT) coated LFP using the hydrothermal method combined with tape coating methodology. The densely packed carbon-coated LFP exhibited excellent Li-ion storage capacity of up to 4.3 V. On the other hand, the half-cell study failed to yield adequate capacity values, which might be attributed to the increased crystallite size (51.07 nm) and shape (Capsule-like) of the synthesized LFP, thereby restricting the ionic mobility and conduction of electrons [24].
The size distribution, shape, temperature, and thickness of the carbon layer influence the production of the secondary surface phase (Fe2P) in LFP [23]. In general, temperature-assisted carbon coatings entails two processes (i) Reduction of carbon to gas (ii) Formation of secondary phases Fe2P and LiPO4. The carbon reduction by smaller particles (less than 20 nm) of LFP may result in improved reactivity because of the increased surface area. In a recent study, it was found that 60 nm thick carbon coating on LFP necessitates high temperature for formation of secondary phases, whereas 2 nm thick carbon coating can form Fe2P phase at low temperature [25]; this study suggests that these additional Fe2P phases facilitate improvements in the electrochemical performance because of its active species. MV Reddy et al. and group designed uniform carbon (30 to 40 nm) coated LFP nanoplates cathodes for LIBs and identified the shorter Li+ diffusion pathway because of the reduced size and plate morphology [26]. The same group also identified the increase in volume and linear expansion coefficient of LFP in accordance with increased temperature (777 °C). 
Secondly, the effect of carbon-coated LFP on Li-ion intercalation, diffusion pathway is a recent topic of discussion. Le et al. identified that CNTs prepared via a hydrothermal route would not affect the Li-LFP intercalation-deintercalation routes but ameliorate the electronic conductivity and ion diffusion velocity [27]. In terms of endeavoring a good perception of LFP intercalation mechanisms, Lu et al. and co-workers reported an X-ray microscopic investigation on Li0.5FePO4 nano- and bulk structures. Using multiple spectroscopic, microscopic tools, and DFT calculations, they found a drastic decline in Li concentration from surface to bulk of Li-rich particles. In contrast, there was no change in the concentration of Li for the Li deficient particles, which reveals the surface sluggish lithiation behavior of Li-rich particles transferred from Li deficient to Li rich bulk particles. Li-rich particles are mapped in red, whereas Li-poor particles are marked in green. The lithiation and movement of ions in the [010] transport plane change slightly since the [010] plane is the fastest Li diffusion pathway in LFP [28].
Finally, the bonding involving a heat-injected coating is significantly stronger than mechanical linkages. Ventrapragada et al. developed a CNT-coated LFP cathode via sustainable spray coating without any surfactants. Carboxymethylcellulose, rubber, graphite and carbon super P were taken as source materials to derive C-LFP cathode, which boosted the cell performance by enhancing the energy density (~460 Wh Kg−1) [29].

Effects of Fabrication Method, Additives, and Surfactant on 1D C/LFP

1D Carbon nanofibers (CNF) with a high specific surface area have an extensively carbonized framework with a short d spacing ((d002) > 0.335 nm) and considerable crystal thickness, allowing for superior mechanical characteristics. Further, multifunctional additives change the design, morphology, and physical traits of CNFs, encompassing a wide range of properties required for electrochemical application. Fongy et al. and group produced CNF and used a non-ionic surfactant to blend them into a slurry of LFP cathode; they estimated the influence of CNF on the ionic and electronic conductivities of LFP and identified that the addition of CNF has enhanced the capacity; they analyzed the impact of CNF on LFP’s electron transfer conductivities and reported that adding CNF boosted the capacity; they further asserted that the porosity of LFP-CNF structure has contributed to improved electronic wiring properties and diffusion of Li+ ions by generating multiple 1D channels [30]. 
Three practices were mostly reported in the past for preparing LFP-carbon composites, namely,
(i)
Mixing of carbon and LFP, post synthesis;
(ii)
Depositing or coating carbon structures over LFP via solution/vapor phase;
(iii)
In-situ co-formation of LFP carbon composite.

Effects of Surface Functionalization on 1D C-LFP

Surface properties and ion conduction of the CNF can be adjusted via the functionalization of –OH, C=O, –COOH groups into the active sites of the CNF. Exterior functional groups primarily function as binding spots and thereby crosslinking or interlinking the molecules during the fabrication and electrochemical testing processes. Activating structures such as CNT, graphene, Mxene, and Prussian blue analogues have been decorated over the CNF-LFP cathodes to widen the pores and increase the surface roughness; this will lead to enhanced ion kinetics favoring sufficient capacity and extended shelf life of the cell. Increasing the reactive spots and changing the hydration tendency of the carbon frameworks on the exterior portion of CNF might increase sorption capacity. Carbon sources like CH4 and C2H2 have typically been utilized to mount CNTs onto CNFs while using vapor deposition methods. The morphological traits of the coated carbon can be altered through catalysts.

2.2.2. Two-Dimensional (2D) C-LFP

Effects of Fabrication Methods and Additives

The easiest and the most direct way to produce LFP-G composites is to manually combine the two materials that are synthesized separately. However, significant improvement in the electrochemical performances were not observed, resulting in decrease in electron conduction. The decrease in electronic conductivity in such circumstances might be due to inadequate interaction among graphene and LFP. 
In situ growth of LFP on pre-produced G has promoted the contact between LFP-G, dispersion strength and reduced aggregate formations, resulting in improved ion diffusion. Wang et al. prepared LFP-G composite via in-situ approach using Li2CO3 and NH4H2PO4 as precursors. He reported that the concentration of G greater than 5 wt% was additives, which generated notable improvements in the discharge capacity [31]. Yang et al. synthesized homogeneously distributed 2D graphene on 3D LFP porous structure via sol gel technique, with an evolution of xerogel framework comprising GO, LiH2PO4 and additives, which gets reduced into G decorated 3D LFP [32].

Effects of Particle Size, Thickness, and Surface Functionalities

Dual coating or compositing of carbon-LFP-G (G-Graphene) structures can improve the electrochemical performance as the overall conductive framework and structural rigidity increases. However, unevenly distributed carbon over LFP or multiple disordered deposition of G/carbon layers may reduce the effectiveness of complete electrode functionality. Further the amount of G composited with LFP has a significant effect on the electrochemical performance. Substantial incorporation of G into LFP has the possibilities of reducing the volumetric energy density due to poor dispersion of G, attributed to large G to smaller LFP ratio and reduces the tap density. The optimal concentration of G in LFP, according to several study groups, is between 2 to 25 wt% [33]. However, it appears that there is no one standard value for comparing these optimal carbon content levels.
The thickness of rGO layer has an impact on the electrochemical performance. Multiple stacking of folded G/rGO layers might result in reduced dispersion of G into LFP and creates aggregates. On the other hand, unfolded stacking of G layers can facilitate the migration of Li+ ions evenly from both the electrode direction, decreasing the polarization, and charge transfer resistance [34]. Though, G/rGO/carbon additives improve electron conduction, their excessive usage may hinder the passage of Li+ ions. Alternatively, highly porous carbon structures have adequate spacing for the migration of Li+ ions between LFP and the active anode material [35]. 
Extrusion method can be utilized to change the electrode thickness (from over 20 µm to a few µm), but recent researchers suggest that the carbon thickness must be kept below 20 µm for high electrochemical performance. Yi Cui and group demonstrated a new technique for producing Li-rGO anode below 20 µm thickness by employing a calendaring process for rGO preparation followed by contact loading of molten Li [36]. The same can be possibly used for preparing LFP-carbon cathodes. 

2.2.3. 3D Carbon LFP Composite

As previously stated, graphene has a large surface area as well as a high electrical conductivity. However stacking of multiple layers and porosity are crucial factors to be considered while designing LFP-G electrodes. Many benefits, such as systematic porosity pathways, excellent electronic conductance, and outstanding mechanical strength, may be found in the 3D carbon nanostructure due to structural interconnections. More research in the realm of 3D C-LFP is required for optimizing the performance characteristics. At present, in situ template synthesis and polymerization reactions aid in fabricating 3D C/LFP. Weng et al. and colleagues used a one-step condensation polymerisation reaction to prepare the precursor, hydroquinone-formaldehyde resin (HFR). Then, NH4H2PO4, Fe(NO3)3, and Li(OH)2 were then mixed with the HFR and subsequently carbonized at 600 °C for obtaining LFP/3D carbon structures. Next, they synthesized 3D carbon/LFP in-situ using glucose as a carbon source for comparison. Due to carbonization and polymerization, a solid 3D structure with higher number of internal pores were reported; these internal pores boost the Li+ ion migration and the electrochemical conversion efficiency. The 3D solid sourced from glucose and HFR polymerization has resulted in the discharge capacity of 151.1 mAh g−1 and 169.3 mAh g−1 at 0.1 C, due to the interconnected porous structure [37]. Porous 3D LFP-G composites were found to increase the conductivity and reversible capacity even at varied current densities [38].

3. Effects of Electrolyte in Performance of C-LFP

Electrolyte type has a significant influence on LIBs performance. Solid-state electrolytes, especially polymer electrolytes popularized by Armand et al. are safer, sustainable, and hold high energy densities [39][40][41][42][43][44]. Perhaps, liquid electrolytes that are electrochemically, mechanically, and thermally stable, paired with a polymer/ceramic separator, could help until solid electrolytes become commercially viable. Armand et al. developed salts such as LiTFSI, TFSI, LiFPFSI and DFTFSI and polymers such as polyethylene oxide (PEO), pentaerythritol tetraacrylate (PETEA) for solid polymer electrolytes (SPE) [14]. Nanosized carbon, Si, and alumina fillers have been shown to increase the ionic conduction of SPE with the conductance augmentation being more beneficial for smaller nanofillers. The clumping of fillers in SE, on the other hand, inhibits ionic charge transport enhancement. Other challenges such as formation of Li dendrites, unstable electrochemical window, and conductivity concerned with SPE, are recapturing to be optimized by the researchers. Integration of ionic liquids into the polymer electrolytes has enhanced the conductivity. 

This entry is adapted from the peer-reviewed paper 10.3390/batteries8100133

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