Research method

X-ray absorption
fine structure (XAFS)

Development of Long-Life Next-Generation Li-IonBatteries
Clarification of the causes of performance deterioration of Li-ion batteries

Role of SPring-8

Achievements

Clarification of the causes of performance deterioration of next-generation Li-ionbatteries* with a Ni oxide cathode at the atomic level by examining the charging
and discharging processes
Optimization of the composition of the cathode to suppress the growth of thedeteriorated layer and to prolong the life of the batteries
Paving the way to applying Li-ion batteries to hybrid and electric cars by theexpected reduction of their weight and cost
R&D facility: Toyota Central R&D Labs., Inc.

*Li-ion battery: a type of secondary battery that can be reused by charging. Li-ion batteries have been increasingly used in mobile electronic
devices because of their lighter weight and greater capacity than Ni-Cd and Ni-MH batteries. Hybrid and electric cars also require long-life
secondary batteries, and various R&D laboratories around the world are competing in the development of Li-ion batteries with improvedperformance.

BackgroundResults

Li-ion batteries in practical use utilize Co as thecathode material. Only a small amount of Co is
produced worldwide, and it is therefore expensive,
which increases the cost of batteries. There is now
a demand for next-generation batteries that use
inexpensive abundant materials.
A material mainly composed of Ni oxide has
recently attracted attention as a new cathodematerial to replace Co. However, this material has
the problems that the structure of the cathodeparticles changes and the battery performance
deteriorates at an early stage with repeatedcharging and discharging. The elucidation of thecauses of these problems is desired.

The SPring-8 high-brilliance X-ray enables us todirectly observe the states of cathode particles. As aresult of observing a battery that was repeatedly
charged and discharged, it was clarified that adeteriorated layer with decreasing transmission ofLi + was grown on the surface of the cathodeparticles and that Ni 2+ block the path of Li +.
From the observation of batteries with different
electrode compositions, we found a composition atwhich the formation of a deteriorated layer can besuppressed. Thus, the life of batteries wassuccessfully prolonged.

Publication: T. Nonaka et al.; Journal of Electrochemical Society 154, A353-A358 (2007)

Principle of Li-ion battery

Mechanism underlying deterioration of battery performance

A Li-ion battery comprises a cathode, an anode, an electrolyte, a separator, and otherelements. The battery performance is determined by the cathode material, and LiCoO2 iscurrently widely used. Graphite (C) is used for the anode and lithium salt for the electrolyte.
During charging, the Li+ in the cathode dissolve in the electrolyte and penetrate into thegraphite structure of the anode. The opposite reaction occurs during discharging.

1) Growth of deteriorated layer
The actual cathode comprises cathode particles of a few µm
diameter (secondary particles) surrounded by an electrolyte, each ofwhich is an aggregate of a number of single-crystal particles (primaryparticles). It becomes increasingly difficult for the surface of thesesecondary particles to pass Li+ upon repeated charging anddischarging.

Current

Reaction inside battery

Lithium ion: Li+Electrolyte: Li salt

Growth mechanism of deteriorated layer
Initial stateCharging

InsideCathodeDischarging
Current

26

Anode

Deterioration ofsurface quality

Electrolyte: Li salt

Formation ofdeteriorated layer

Charge-dischargecycle2) Deteriorated layer blocking path of Li+
From the atomic-level observation, it was found that the path of Li+ isblocked by Ni2+ that replace some of the Li+ upon repeated charging
and discharging.

+Lithium ion: Li

Electrolyte: Li salt

2+Ni

InsideSecondary particles in cathode: a few µm in diameter27