Graphene conductive additives for lithium ion batteries: Origin, progress and prospect

Abstract

This paper gives a critical review on the scientific origin, current research progresses and application prospects of graphene conductive additives applied in lithium-ion batteries (LIBs) and emphasizes that the electron transport (enhancement of electron conductance by graphene) and ion diffusion (steric effect of graphene for ion diffusion) should be considered comprehensively for a real mass application. Due to the high electronic conductivity, two-dimensional planar geometrical structure and the ″most flexible and thinnest″ character, our group proposed that graphene is a very promising conducting additive for LIBs. Through ″plan-to-point″ contact model with active material particles, graphene can improve the electronic conductivity of the electrode laminate with much less addition fraction, and hence enhance the energy density of the LIBs. Therefore, graphene shows much better performance than other types of conductive additives, such as carbon black, conducting graphite and even carbon nanotube. Due to the size difference of active material particles, the optimized addition fraction of graphene is different. For example, 2 wt% of graphene works best for LiFePO4, while the most suitable addition amount of graphene in LiCoO2 is only 1 wt%. At the same time, graphene and conducting carbon black can work together to construct a much more efficient conductive network. This kind of binary conductive additive can build high speed pathway for electron both in long and short distance (that is, electrons can transport in the whole electrode and gain access to the whole surface of the active material particle simultaneously), and further improve the electrochemical performance of active materials. Hence, hybrid material containing graphene and carbon black is fabricated to use directly as a novel binary conductive additive. Other than the improvement of the electron conduction, the introduction of graphene brings a bottleneck for its application in LIBs, which has been reported by our group in 2012 based on a 10 Ah LIB using graphene as conductive additive, that is, the steric effect for lithium ion diffusion. Ion diffusion in the porous electrode will be retarded by planar graphene because it is very hard to penetrate through the hexagonal carbon ring for lithium ions. Many efforts have been made by our group to illustrate this effect. We found that it is the thickness and tortuosity of the electrode laminate that determines and the situation varies with different active material. When it comes to LiFePO4, the ion steric effect is not obvious when the electrode is thin (e.g. thinner than 26 μm). When the electrode sheet is thicker (>39 μm), the rate performance of LiFePO4 becomes worse when the graphene addition increases. However, when it comes to LiCoO2, which is large in particle size, the steric effect from graphene disappears. It can be attributed to the pore tortuosity of the electrode. Since the particle size of LiCoO2 is much larger, the electrode is not as tortuous as that of LiFePO4 electrode. Therefore, ions diffuse easily along the electrode thickness direction. Nevertheless, it should be noted that the steric effect in LiFePO4 electrode can be elimated by introducing pores on the surface of graphene or making ribbon-like graphene to provide more diffusion path for lithium ions in the porous electrode. From the practical points, the dispersion of graphene in the active materials is a vitally important but a tough job. In order to construct an efficient conducting network, graphene should be well dispersed and cover every particle of the active material in the electrode. Agglomeration of graphene will not construct a good conducting network, and even bring adverse effects for the ion diffusion. Normal mixing equipments alone cannot ensure good dispersion of graphene, and ultrasonication is an efficient pre-treatment step. New techniques with higher dispersion efficiency are expected to realize really uniform and monolayered dispersion of graphene in active materials, which is very important for mass application of graphene additives in LIBs.