With the continuous development of research, high-performance lithium battery electrode materials are emerging in an endless stream. Graphene's high electrical conductivity, high thermal conductivity, high specific surface area, and many other excellent properties have significant theoretical and engineering value in solving this problem.
Advantages of direct lithium storage using graphene:
1) High specific capacity: Lithium ions exhibit non-stoichiometric intercalation-deintercalation in graphene, resulting in a specific capacity of 700~2000 mAh/g;
2) High charge/discharge rate: The interlayer distance of multilayer graphene materials is significantly greater than that of graphite, which is more conducive to the rapid insertion and extraction of lithium ions. Most studies also show that the capacity of graphene anodes is around 540 mA·h/g, but the rate performance is also greatly affected by the decomposition of a large number of oxygen-containing groups on its surface during charge and discharge or their reaction with Li+, which causes the battery capacity to decay.
The defects introduced by heteroatom doping can change the surface morphology of graphene anode materials, thereby improving the wettability between the electrode and the electrolyte, shortening the distance of electron transfer inside the electrode, increasing the diffusion and transfer rate of Li+ in the electrode material, and thus improving the conductivity and thermal stability of the electrode material.
However, using graphene materials directly as battery anodes still has some drawbacks, including:
1) The prepared monolayer graphene sheets are very easy to stack, and the reduction in specific surface area causes them to lose some of their high lithium storage capacity;
2) The initial coulombic efficiency is low, generally below 70%. Due to the large specific surface area and abundant functional groups, the electrolyte decomposes on the graphene surface during cycling, forming an SEI film; simultaneously, the residual oxygen-containing groups on the carbon material surface undergo irreversible side reactions with lithium ions, causing a further decrease in reversible capacity.
3) Rapid initial capacity decay;
4) Voltage plateau and voltage hysteresis. Therefore, to solve these problems, combining graphene with other materials to create graphene-based composite anode materials has become a hot topic in lithium battery research and a direction for the development of lithium battery anode materials.
For lithium-ion battery anode materials, transition metal oxides or promising Si-based materials, after graphene doping, have already demonstrated excellent electrochemical properties in terms of specific capacity, voltage characteristics, internal resistance, charge-discharge performance, cycle performance, and rate performance. Heteroatom doping in graphene introduces more surface defects, improving the conductivity of graphene materials and resulting in composite materials with superior performance.
