Effects of tin doping on physicochemical and electrochemical performances of LiFe1−xSnxPO4/C (0 ≤ x ≤ 0.07) composite cathode materials

Nanocrystalline LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples are synthesized using SnCl4·5H2O as dopant via an inorganic-based sol–gel method. The dependency of the physicochemical and electrochemical properties on the doping amount of tin are systemically worked out and regular changes are revealed. In the whole concentration range, the chemical valence of Fe2+ is not basically changed whereas tin is found in two different oxidation states, namely +2 and +4. The replacement of Fe2+ by supervalent Sn4+ would lead to electron compensation. Under the synergetic effects between the charge compensation and the crystal distortion, the electrical conductivities for the bulk samples first increase and then decrease with the increasing amount of Sn doping. Upon the doping amount, the apparent lithium-ion diffusion coefficient and the electrochemical performance also display the similar trends. The doping is beneficial to refine the particle size and narrow down the size distribution, however optimizing the doping amount is necessary. Compared with other samples, the sample with a doping amount of about 3 mol% delivers the highest capacities at all C-rates and exhibits the excellent rate capability due to the high electrical conductivity and the fast lithium-ion diffusion velocity.

► The dependency of the physicochemical and electrochemical performances of the nanocrystallized LiFePO4 derives on the doping amount of Sn in the concentration range of 0 to 7 mol% was systematically investigated and the corresponding relationship was well established.
► The mixed-valence doping mode of Sn was firstly observed by XPS.
► The charge compensation mechanism of aliovalent cation Sn4+ doping was deduced by ICP-OES and defect chemistry.
► The Sn doping is beneficial to refine the particle size and narrow down the size distribution; however, optimizing the doping amount is necessary.
► Upon the doping amount, the electrical conductivity for the bulk sample and the lithium-ion diffusion coefficient varies nonlinearly with the doping amount of Sn. And the electrochemical performances also show similar changing trends.