More details on work supporting NEC prototype 4.5V Li-ion battery; modified LiNiMnO cathode and fluorinated electrolyte
NEC Corporation has developed a prototype next-generation manganese lithium-ion battery featuring cathodes that support higher voltage operations (4.5V rather than 3.8V) and an electrolyte solution that improves the stability of the higher voltage operations. (Earlier post.) The new cathode and electrolyte solution improve battery energy density by approximately 30%.
At PRiME 2012 (Pacific Rim Meeting on Electrochemical and Solid-State Science), in Honolulu, a team from NEC’s Smart Energy Research Laboratories led by Takehiro Noguchi presented a paper describing work on cathode materials and electrolytes enabling the higher-capacity prototype cell.
LiNi0.5Mn1.5O4 is a promising next-generation cathode material that has attracted the interest of multiple research teams because of its large capacity and its high charge-discharge potential of around 4.65V versus Li.
However, because of the higher potential than offered by more conventional cathode materials, the 5V-class spinel encounters the problems of decomposition of the electrolyte and manganese dissolution, resulting in capacity fade. Decomposition of the electrolyte may cause gas generation, and it can cause swelling of cells.
Earlier, Noguchi and his colleagues had improved the lifetime of the cathode material by substituting titanium (Ti) for a portion of the manganese. To suppress electrolyte decomposition, they sought to develop electrolyte solvents which have higher resistance to oxidation—itself another area of substantial research interest.
In the work presented at 2012 PRiME, they examined the use of a fluorinated phosphate ester— tris(2,2,2-trifluoroethyl) phosphate (TTFEP)—as an electrolyte solvent.
With LiNi0.5Mn1.5-xTixO4 as the cathode and graphite as the anode material in stack type laminated cells, the team used two electrolytes: a 1 M solution of LiPF6 in 30:70 (v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC); or in a mixture of EC and TTFEP.
Charging was carried out at constant current of 1 C up to 4.75 V and constant voltage at 4.75 V. Discharging was performed with constant current of 1 C until 3.0 V. The cycle tests were performed at 20 °C and 45 °C.
Capacity retentions with the different electrolytes were both about the same at around 77%. However, the increase rate of cell volume after 150 cycles at 45 °C for the EC/DMC electrolyte was 67%, while the increase rate for EC/TTFEP was 2%. In other words, the amount of gas generation was found to be significantly reduced by replacing DMC with TTFEP.
Takehiro Noguchi, Makiko Uehara, Yukoh Katoh, Hideaki Sasaki and Koji Utsugi (2012) Examination of applying fluorinated phosphate ester to electrolyte solvent of the batteries with LiNi0.5Mn1.5-xTixO4 cathode. Honolulu PRiME 2012
WO/2012/127717: Secondary Battery