Development of High-Energy Lithium-Ion Batteries through the Anode-Side Substitution of Graphite By Si/C-Composite

Abstract

In the recent years, rechargeable lithium-ion batteries (LIBs) have gained in importance for electronic devices and electric vehicles. Thus, research and development focuses on improving energy and power densities as well as durability of LIBs. Especially for high energy and power densities, the electrode materials must possess a high specific storage capacity and a high coulombic efficiency. However, state-of-the-art anode and cathode electrode materials, e.g. graphite and LiFePO4 exhibit a high coulombic efficiency but a rather low theoretical storage capacity (372 and 170 mAh⋅g-1, respectively). In the last decade silicon has become a promising anode material due to its high theoretical specific capacity of 3579 mAh⋅g-1at ambient temperature. However, this high specific storage capacity owing to host up to 3.75 lithium atoms per silicon atom leads to extreme volume expansion up to 300 % during lithiation, which results in pulverization and delamination of the electrode material after few cycles. Various approaches have been conducted to overcome these issues e.g. by using nano-sized active material or carbon coated silicon composite material. In addition to the materials science the electrode structure is of particular importance for the electrochemical performance. Electrode composition, binding mechanism due to the use of suitable binder polymers, particle size distribution of the active material or the modification of the SEI are some exemplary parameters to stabilize the electrode structure and to handle such high mechanical stress during lithiation/delithiation. Finally, the developed silicon anode must be implemented into a full cell by combining the anode with a suitable cathode. Because in a commercial LIB only the cell voltage is controllable, the electrode balancing of the full cell, that means the capacity ratio of the negative to positive electrode (N/P ratio) and the electrochemical voltage window in which the full cell is operated, is practically important to achieve a long cycle life and a high coulombic efficiency for the battery. For example, it can be ensured by adjusting the electrochemical potential window and choosing a well-balanced electrode design (normally N/P>1) that irreversible capacity losses can be prevented, which are correlated to lithium plating on the anodes surface during the charging process. Here we present electrochemical investigations of high capacity and high efficiency graphene coated silicon nanocomposite based electrodes prepared by using a water based wet chemical doctor blade manufacturing process. The commercially available Si/C-composite is mixed with graphite to obtain a Si/C-anode that provides a capacity of 1000 mAh⋅g-1 with an average coulombic efficiency >99 % over more than 500 cycles in half cells. The developed Si/C-anode is combined with a LiFePO4-cathode to build high-energy full cells. Investigations focus on the influence of the N/P ratio and the voltage window on the electrochemical performance of Si/C-LiFePO4 full cells. It will be shown that the developed Si/C-anode improves the energy density of the full cell regarding to a comparable C6-LiFePO4 full cell over more than 200 cycles.