Conversion anodes, for example, transition metal oxides (Fe
3O
4, Co
3O
4, CuO, etc.).
-
Figure 1 illustrates the advantages and drawbacks of these three types of materials and the connection between working potentials and the specific capacity of the anode materials
[25]. Generally, depending on the anode active material, polymer coating on the particles could help resolve some critical challenges, such as poor cycle life and C-rate capability, low Coulombic efficiency (CE), unstable SEI, and high irreversible capacity, which will be addressed here in detail for the specific polymers and anode materials.
Figure 1. Anode active materials with three different primary electrochemical (de)lithiation mechanisms: (a) Radar plot comparing five critical categories of capacity, cost, cycle life, safety, and power. (b) Schematic illustration comparing potential vs. capacity of certain anode materials.
A thin polymer layer on active anode materials could act as an artificial SEI. This layer can be fabricated for mechanical flexibility to maintain the passivation of active anode materials. The polymer film could be synthesized via different techniques
[26,27,28,29,30,31,32,33,34,35,36,37,38,39][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Generally, thin polymer films could accommodate volume expansion (unlike glass or ceramic layers) while simultaneously demonstrating good chemical and structural stabilities during (de)lithiation processes. The polymer film thickness is typically about 2–25 nm
[10,40,41,42,43,44,45,46,47,48,49,50,51,52,53][10][40][41][42][43][44][45][46][47][48][49][50][51][52][53].
Conductive polymers are attractive additive materials for LIBs because of their outstanding electrochemical properties: enhancing the electronic conductivity, inhibiting the phase transition, increasing structural stability, decreasing active material dissolution, leading to a remarkable improvement in reversible capacity, rate capability, and cycle stability. Conductive polymers, such as polypyrrole (PPy)
[44], polyaniline (PANi)
[54], poly(3,4-ethylenedioxythiophene) (PEDOT)
[55], PEDOT:poly(styrenesulfonate) (PEDOT:PSS)
[31], and polythiophene (PT)
[33], but also other polymers, such as polyvinylidene fluoride (PVDF)
[12,32][12][32] and Polydopamine (PDA)
[56], have been used as attractive coating agents for active anode materials to improve the mechanical flexibility and the electrochemical performance of LIBs.
Figure 2 demonstrates the contribution of polymers reported in the literature chosen here for coating active anode materials. The category “others” comprises the literature using polyvinylpyrrolidone and polyacrylonitrile
[49], poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane)
[51], poly(vinyl alcohol)
[52[52][57],
57], polyether, polyethylene glycol tert -octylphenylether and polymer polyallyl amine
[17], poly(diallyl dimethylammonium chloride) and poly(sodium 4-styrenesulfonate)
[58], polyacrylic acid and polymethacrylic acid
[9], PVDF
[12[12][32],
32], 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane
[13], PEDOT:PSS
[31,59][31][59], PDA
[56], poly(dimethyldiallylammonium chloride) and poly(methyl methacralate) (PMMA), poly(sodium-p-styrenesulfonate)
[60], 1,3,5-trimethylcyclotrisiloxane (V3D3)
[13] and poly(ethylene oxide)
[61]. These polymers may serve as a host for Li-ion (de)intercalation and enhance the electron transfer in the electrode, particularly with the electrodeposition method. In this
pape
r, wentry, researchers will discuss the effects of polymer coating on the electrochemical performance of anode materials.
Figure 2.
The percentage of various polymers (PPy, PEDOT, PT, PANi, and others) that have been used to coat anode materials so far reported in the literature.