The Lithium Ion Battery

New generations of lithium-ion batteries are required combining high energy and power densities with low cost and high safety, for applications such as electric vehicles or static energy storage. This in turn, requires the introduction of new electrode materials. It has been demonstrated that when extra lithium is added to layered LiMO2 electrodes, notably those in which M = Mn & Ni, or Mn, Ni & Co, capacities > 200 mA h g-1 can be obtained if the electrodes are initially charged to above 4.5 V. Of particular importance are the lithium-rich Li-Mn-Ni-Co-O materials that are being exploited for the next generation of high-energy lithium-ion cells. The composition of these Li-rich electrodes can be represented either in conventional layered notation, Li1+δM1-δO2, or in two-component notation, xLi2MnO3•(1-x)LiMO2, because the additional Li+ ions (δ) are charge-compensated by Mn4+ to form a Li2MnO3 component which is structurally integrated with the residual LiMO2 component. We demonstrated in a combined neutron-diffraction/in-situ mass spectroscopy study that, when charged to 4.6 V and higher, the Li2MnO3 component is electrochemically activated by the removal of lithium and oxygen (a net Li2O loss), thereby yielding an electrochemically-active MnO2 component, without damaging the overall layered character of the electrode.

A. R. Armstrong, M. Holzapfel, P. Novák, C. S. Johnson, S-H. Kang, M. M. Thackeray and P. G. Bruce, "Demonstrating oxygen loss and associated structural reorganization in the lithium battery cathode Li[Ni0.2Li0.2Mn0.6]O2", J. Amer. Chem. Soc. 2006, 128, 8694.

Recently, a new class of polyoxyanion cathodes based on the orthosilicates, Li2MSiO4 (where M = Mn2+, Fe2+, Co2+), has been attracting significant attention. The relatively strong Si-O bonds promote similar lattice stabilization effects to the phosphate bonds found in the much-studied LiFePO4. Of these silicates, the most studied is Li2FeSiO4, with iron and silicon being among the most abundant and lowest cost elements. We have established the crystal structure of cycled Li2FeSiO4 using powder neutron diffraction and explored the Li+ migration pathways using atomistic simulation techniques in collaboration with the group of Prof. M. Saiful Islam at the University of Bath.

A. R. Armstrong, N. Kuganathan, M. S. Islam and P. G. Bruce, "Structure and lithium transport pathways in Li2FeSiO4 cathodes for lithium batteries", J. Amer. Chem. Soc. 2011, 133, 13031.

Lithium can be reversibly intercalated into layered Li1+xV1-xO2 (LiCoO2 structure) at ~0.1V, but only if x > 0. The low voltage combined with a higher density than graphite results in a higher theoretical volumetric energy density; important for future applications in portable electronics and electric vehicles. We have shown that Li+ intercalated into tetrahedral sites are energetically more stable for Li-rich compositions, since they share a face with Li+ on the V site in the transition metal layers. Li incorporation triggers shearing of the oxide layers from cubic to hexagonal packing because the Li2VO2 structure can accommodate two Li per formula unit in tetrahedral sites without face sharing. Such understanding is important for the future design and optimisation of low-voltage intercalation anodes for lithium batteries.

A. R. Armstrong, C. Lyness, P. M. Panchmatia, M. S. Islam and P. G. Bruce, "The lithium intercalation process in the low-voltage lithium battery anode Li1+xV1-xO2", Nature Mater. 2011, 10, 223.

Titanates are being intensively investigated as anodes for lithium-ion batteries due to their superior safety and rate capability compared with graphite, although their higher voltage lowers the overall energy density of the lithium-ion cell. TiO2 possesses twice the theoretical specific capacity (335 mA h g-1) compared with Li4Ti5O12 (175 mA h g-1), i.e., is comparable to graphite, rendering TiO2 potentially attractive as an anode for Li-ion batteries. TiO2(B) can accommodate more lithium than any other TiO2 polymorph as a bulk material (micrometer-sized particles). It has been shown that nanostructured forms of TiO2(B) enhance rate capability compared to the bulk, with nanoparticulate TiO2(B) exhibiting the highest rate capability to date.

Y. Ren, Z. Liu, F. Pourpoint, A. R. Armstrong, C. P. Grey and P. G. Bruce, "Nanoparticulate TiO2(B): an anode for lithium-ion batteries", Angew. Chem. Int. Ed. Engl. 2012, 51, 2164.