Higher capacities and energies are needed in order to achieve widespread acceptance of Li-ion batteries as power sources in electric vehicles. In principle, Li-ion batteries offer two possibilities to achieve higher specific energies. Since the batteries’ energy is determined by the operational voltage and the active materials capacity, one could improve either the capacity of the active materials or the operating voltage of the cell.

Both approaches are pursued by many research groups. Higher capacities can for example be achieved with Li-rich layered oxides, where the use of more than 1 Li in the active material can considerably improve the capacity. The operational voltage is mainly determined by the potential of the cathode material vs. Li-metal. Therefore high voltage cathode materials
(e.g. LiM_xMn_2-xO₄, with M = e.g. Ni, Co) are used.

However both approaches suffer from different limitations and problems. The need for balancing and the commonly relatively low mean voltages of ca. 3.5 V vs. Li⁺/Li⁰ reduce the gain in specific energy for materials with high capacities. On the other hand, the use of high voltage materials leads to unwanted site reactions and severe capacity fade when applied with a graphitic Anode. This is because the cells are operated above the stability window (ca. 1.0 – 4.3 V vs. Li⁺/Li⁰ in the presence of transition metal ions like Ni⁴⁺ and Mn⁴⁺) of current organic electrolytes. Furthermore the theoretical capacity of 147 mAh g^-1 for the LiNi_0.5Mn_1.5O₄ high voltage spinel is relatively low.

The shortcomings of the high voltage spinel can be overcome, when metallic lithium is used as the anode material. Not only the cycling stability is significantly improved but also the use of metallic lithium leads to the ability of cycling of 2 Li eq. per cycle, since the spinel lattice is able to host 2 Li eq. This is leading to a theoretical capacity of 294 mAh g^-1 and therefore a theoretical specific energy of ~1030 Wh kg^-1 can be reached when the cell is cycled between 2.0 and 5.0 V vs. Li⁺/Li⁰.

Unfortunately, the cycling of 2 Li eq. is leading to a severe capacity fade due to a phase transition from cubic to tetragonal in the low voltage region associated with material degradation.

In this work doping with transition metal ions is used to improve capacity retention when cycling between 2.0 and 5.0 V vs Li⁺/Li⁰. Initial capacities and stabilities are directly dependent on synthesis conditions and doping elements. The goal is to achieve a high stability of the material at a low tradeoff of capacity. Therefore different combinations of doping elements are used and tested at different cycling conditions. In addition to the transition metal ions, fluorine doping is also used to increase cycling stability.

The influence of different synthesis, doping and cycling conditions on cycling stability and capacity retention are discussed.