Lithium-oxygen batteries hold high promise as the next generation high energy density battery technology due to their high specific energy and theoretical capacity.1 The electrochemistry in a lithium-oxygen battery is highly sensitive to the quality of oxygen supplied at the cathode. Presence of moisture and carbon dioxide degrades the battery performance by accelerating the electrolyte decomposition.2 We previously reported the working of a closed system lithium-oxygen battery using a Li2O2-carbon sandwiched cathode.3 Our current work reports a chemically synthesized lithium peroxide-carbon nanofiber (Li2O2-CNF) composite that can be used as cathode in the closed system. Oxygen is generated during charge which is stored in the CNF matrix, and it is converted back to Li2O2 during discharge. This eliminates the need for oxygen extraction and purification systems that would be required for a lithium-“air” battery.
Experimental
The cathode consists of binder-free Li2O2-CNF paper. It was obtained by chemically synthesizing Li2O2 in a methanol solution containing dispersed self-weaving carbon nanofibers. The mixture was vacuum filtered to obtain the binder-free paper containing approximately 35 wt.% Li2O2 and the Li2O2 loading is approximately 1-1.5 mg cm-2.
Results and Discussion
The cathode contains Li2O2 particles embedded in the CNF matrix providing excellent electrical contact. As can be seen in the voltage profile in Fig. 1a, the Li2O2 is activated at nearly 4 V and the oxygen evolution reaction proceeds till 4.3 V where almost all of Li2O2 is completely charged. The first discharge converts the available oxygen into nanometer sized Li2O2 that is evenly distributed across the electrode, as shown in Fig. 1b. Following cycles exhibit a 3.5 V plateau wherein oxygen evolution dominates and a 4.2 V plateau where electrolyte decomposition and CO2 evolution from Li2CO3 dominate.4 Steady electrolyte decomposition during cycling leads to Li2CO3 formation that results in degraded performance beyond 20 cycles, as shown in Fig. 1c. Use of capacity limited charging to utilize the electrochemical conversion of Li2O2 to oxygen at 3.5 V prolongs the cycle life as the electrolyte decomposition is minimized at the lower voltage. Over 50 cycles can be obtained, as shown in Fig. 1d.
In summary, a binder-free composite cathode with Li2O2 particles intimately bound in the CNF matrix was synthesized. This cell exhibits low overpotentials owing to small particle size. Use of a charge capacity control method that operates primarily in the lower voltage plateau can significantly improve cycle life. Such cathodes can be used with non-lithium-metal anodes and an improved electrolyte that could lead to the development of safe, high capacity lithium batteries.
References
1. G. Girishkumar, B. McCloskey et al., J. Phys. Chem. Lett. 1, 2193 (2010).
2. A. C. Luntz and B. D. McCloskey, Chem. Rev. 114, 11721 (2014).
3. A. Bhargav and Y.-Z. Fu, J. Electrochem. Soc. 162, A1327 (2015).
4. S. Xu, S. Lau and L. A. Archer, Inorg. Chem. Front. 2, 1070 (2015).