The present invention relates to a negative electrode for lithium secondary battery, the negative electrode comprising a current collector and an active material layer carried on the current collector, the active material layer comprising particles grown like columns (columnar particles), and the columnar particles including silicon as a constituent element.
With the development of portable devices such as a mobile phone and a personal computer in these days, a demand for a battery as a power source for such devices is increasing. In a battery for such devices, the usage at ordinary temperatures is demanded, as well as a high energy density and excellent cycle characteristics.
For such demands, a new active material with a higher capacity has been developed for a positive electrode and for a negative electrode, respectively. Among these newly developed active materials, the simple substance, an oxide, or an alloy of silicon (Si) or tin (Sn), which enables a far higher capacity, has been expected as a potential negative electrode active material.
The problem in using silicon as a negative electrode active material is deformations of the negative electrode. When charging and discharging, the negative electrode active material greatly expands and contracts by absorption and desorption of lithium (Li), to greatly deform or distort the negative electrode, and to cause an undulation or winding (wrinkling). Thus, a gap is created between the negative electrode and a separator, causing non-homogeneous charge and discharge reactions, leaving a concern for a decline in cycle characteristic.
For such problems, there has been proposed to suppress the decline in cycle characteristics by providing a space to alleviate the stress from the expansion of the active material in the negative electrode to avoid deformation and winding. For example, Japanese Laid-Open Patent Publication No. 2003-303586 proposes to form columnar particles of silicon on the current collector. Japanese Laid-Open Patent Publication No. 2004-127561 proposes the pattern formation in which an active material capable of forming an alloy with lithium is regularly aligned on the current collector.
In both Japanese Laid-Open Patent Publication No. 2003-303586 and Japanese Laid-Open Patent Publication No. 2004-127561, a negative electrode active material is formed as a columnar structure standing upright along the normal direction of the sheet-like current collector. Thus, a part of the positive electrode active material faces the exposed surface of the negative electrode current collector, not facing the negative electrode active material. Therefore, lithium supplied from the positive electrode active material at the time of charging tends to deposit on the exposed part of the negative electrode current collector, without being absorbed by the negative electrode active material. As a result, lithium cannot be released from the negative electrode effectively at the time of discharging, thereby declining charge and discharge efficiency.
Additionally, at the time of discharging, only the positive electrode active material facing the negative electrode active material is more likely to react. This may cause a partial over-discharge in the positive electrode active material, while declining substantial discharge capacity. When the repetitive charge and discharge cycle further advances the non-homogeneous reactions, the positive electrode active material that could not be discharged will easily be in the over-charged state, while increasing the positive electrode active material under the over-discharged state. Thus, side reaction increases, discharge capacity decreases, and deterioration of battery advances. Especially, when a high-rate charge and discharge is carried out at a large current value, cycle characteristics decline drastically.