Science and technology
How feasible is the graphene + lithium battery?
It is well known that graphene has many excellent properties such as high conductivity, high thermal conductivity, high specific surface area, high strength and rigidity, and has been widely used in many fields such as energy storage, optoelectronic devices, and chemical catalysis.
Lithium-ion batteries are by far the highest energy secondary batteries, but they need to be further increased in energy ratios when used in new energy vehicles. The emergence of graphene has brought about a breakthrough in the high-performance of lithium-ion batteries, which has created a new round of research for high-capacity, high-rate, long-life lithium-ion battery materials.
At present, the research on graphene in lithium battery is mainly divided into two pieces.
The first is to apply on traditional lithium batteries, the purpose is to improve and improve the performance of lithium batteries, such batteries will not have a disruptive effect;
The second is based on graphene to make a new system of batteries, it is a new series, is subversive in performance, called "super battery."
Application of graphene in cathode materials
The positive electrode materials of lithium batteries such as LiCoO2, LiMn2O4 and LiFePO4 are all poor electronic conductors, and their electrical conductivity is 10-4, 10-6 and 10-9 Scm-1, respectively. In the current lithium ion battery system, the positive and negative materials used in the battery itself have low ion and electron conductivity, which is a major factor affecting and limiting the charge and discharge cycle and rate performance of the lithium battery. Therefore, in order to fully utilize the positive electrode material in the charging and discharging process and at the same time improve the rate performance of the battery, a conductive agent is added to the positive electrode material, and the conventional conductive agent is generally graphite. While graphene itself has a very high electron conductivity, the use of graphene as a conductive additive is the most direct and widely used in lithium batteries.
The problem of graphene as a conductive agent
For the practical application of graphene conductive agent, it is necessary to comprehensively consider the "face-to-point" promoting effect of graphene on electronic conductance and the "steric effect" on ion conduction; considering the amount of conductive agent and the energy/power density of the final battery Design the thickness of the electrode. For the lithium ion battery of the LFP system, since the influence of graphene on lithium ion transmission is very strong, it is necessary to pay special attention to the thickness of the electrode.
Application of graphene in anode materials
At present, the anode material commonly used for lithium batteries is graphite, and the advantages of using graphene as a negative electrode material are:
(1) Graphene has good electrical conductivity and corrosion resistance, and it can be used as a negative electrode material to enhance the conductivity of the active material and the current collector;
(2) Graphene sheet layer as a single layer two-dimensional structure, in principle, there is no volume expansion, so the structure is stable, charging and discharging is fast, and the cycle performance is good;
(3) In-situ synthesis of nanoparticles on the surface of graphene to form a matrix composite, by controlling the size of the growth particles, thereby shortening the diffusion distance of lithium ions and electrons, and improving the rate performance of the material;
(4) The nanoparticles uniformly cover the surface of the graphene, which can prevent the polymerization of the graphene sheets and the immersion of the electrolyte into the graphene sheets to a certain extent, resulting in failure of the electrode material.
Graphene directly used as a negative electrode material
(1) Graphene is easy to react with an electrolyte to form a large amount of SEI film due to its small size and high specific surface area, resulting in a large loss of irreversible capacity.
(2) Graphene is prone to agglomeration in the electrode cycle, and agglomeration is irreversible due to van der Waals force, resulting in difficulty in lithium insertion and battery capacity decay.
(3) Graphene is prone to re-stacking during the preparation process, which is demanding for dispersion and drying conditions, resulting in an increase in cost.
(4) Graphene is present, and the application of battery anode material is characterized by low efficiency for the first time and poor cycle performance.
At present, the application of graphene composite materials in lithium batteries has become a hot research topic. How to improve the preparation technology of high quality graphene, find a controllable and large-scale preparation method of graphene, and prepare graphene-based composite materials with excellent performance. Is the focus of current research. If the graphene-based electrode material is used in the field of high-energy density and high power density required lithium-ion battery, it will greatly enhance the comprehensive performance of the power battery and promote the development of electric vehicles and power tools.