Li-ion batteries have higher energy density than Ni-Cd, and require minimal maintenance over their lifetimeOne-pot method could overcome the limits of spinel-based electrodes
Lithium ion-batteries (Li-ion) have become the battery of choice across a range of industries, and in consumer electronics like mobile phones, they have almost entirely replaced nickel-cadmium and nickel metal hydride batteries. And it’s not all that surprising – as well as being rechargeable, Li-ion batteries have higher energy density than Ni-Cd, and require minimal maintenance over their lifetime. They have the potential to be further improved, though, largely through optimising their cathode material.
One promising candidate is lithium manganese spinel (LiMn2O4, or LMO) – it forms a 3D structure that allows for rapid ion (Li+) flow, and is more thermally-stable than cobalt-based cathode materials. Its main drawbacks are its relatively low capacity, and poor cycle performance, which has limited its use to a small number of specific applications. But, a team of Chinese researchers has developed a simple fabrication process for a new spinel-based nanocomposite that may well help overcome these limitations.
Writing in a recent issue of Materials Today Advances [DOI: 10.1016/j.mtadv.2018.12.001], they report on a low-temperature, ‘one-pot’ process for synthesising an LMO-reduced graphene oxide composite. They started with graphene oxide dispersed in alcohol and water. An aqueous solution of MnSO4 was gradually added and the mixture stirred for several hours. Aqueous lithium hydroxide was added next, and after heating for 12 hours, a dark, solid compound was produced. This product was filtered, washed and dried, before finally being calcinated for 2 hours. High-resolution transmission electron microscopy and X-ray diffraction analysis showed that the product was LiMn2O4@rGO (referred to as LMG in the paper).
To test the electrochemical performance of the LMG material, the team needed to incorporate it into a simple battery architecture. So, they coated an aluminium foil current collector with a slurry of 80 wt% LMG, before drying it to form a cathode. This was assembled into a coin cell battery with a lithium anode and a standard electrolyte separator. To act as comparisons, two other batteries were made – one with a pure LiMn2O4 electrode, and the second with ‘physical mixing LMG’ – a form of the nanocomposite in which graphene oxide was simply mixed with pure LMO.
A range of tests were carried out on these electrodes – cyclic voltammetry showed that the LMG cathode displayed the fastest lithium-ion transport and highest reversibility of the three test materials. The authors attribute this to the “intimate interactions” between the graphene oxide nanosheets and the LMO nanoparticles, achieved through their synthesis process. Though the particles themselves are electrically-insulating, by growing them in-situ on highly-conductive substrates, they form a complex network that allow lithium ions to move more easily, improving the electrode’s cycling performance. The LMG electrode also displayed the lowest polarization, and the lowest charge-discharge resistance. Its first-cycle Coulombic efficiency was found to be 97%, and both before and after 150 cycles, the LMG electrode showed the largest exchange current density.
The authors say that, to the best of their knowledge, “…this is the first report of the graphene-based LiMn2O4 nanocomposite, obtained through a soft chemical synthesis route instead of (a) solid-state mixing process or reaction, (and) which was applied as a cathode material.”
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Yinghao Chen, Yulan Tian, Yunzhong Qiu, Zhifang Liu, Huanhuan He, Baojun Li, Huaqiang Cao. “Synthesis and superior cathode performance of sandwiched LiMn2O4@rGO nanocomposites for lithium-ion batteries” Materials Today Advances 1 (2019) 100001. DOI: 10.1016/j.mtadv.2018.12.001