Interests in energy storage technologies have been increasingly higher recently. As applications are expanded to energy of mobile phones, camcorders and notebook PCs, and furthermore, to electric vehicles, efforts on the research and development of electrochemical devices have been more and more materialized.
Electrochemical devices are fields receiving most attentions in such aspects and among these, development of secondary batteries capable of charge and discharge have been the focus of attention, and in developing such batteries, research and development on the design of new electrodes and batteries for enhancing capacity density and energy efficiency have been recently progressed.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990s have received attentions with advantages of having high operating voltage and significantly higher energy density compared to conventional batteries such as Ni-MH, Ni—Cd and sulfuric acid-lead batteries using an aqueous liquid electrolyte.
A lithium secondary battery has a structure of an electrode assembly including a positive electrode, a negative electrode and a separator interposed between the positive electrode and the negative electrode being laminated or wound, and is formed by embedding the electrode assembly in a battery case, and injecting a non-aqueous liquid electrolyte thereinto. The lithium secondary battery produces electric energy through oxidation and reduction reactions occurring when lithium ions are intercalated/deintercalated in the positive electrode and the negative electrode.
In a common lithium secondary battery, a negative electrode uses lithium metal, carbon and the like as an active material, and a positive electrode uses lithium oxides, transition metal oxides, metal chalcogen compounds, conductive polymers and the like as an active material.
Among these, a lithium secondary battery using lithium metal as a negative electrode mostly attaches lithium foil on a copper current collector, or uses a lithium metal sheet itself as an electrode. Lithium metal has low potential and high capacity, and has received much attention as a high capacity negative electrode material.
When using lithium metal as a negative electrode, electron density non-uniformization may occur on the lithium metal surface when operating a battery due to various reasons. As a result, a branch-shaped lithium dendrite is produced on the electrode surface causing formation and growth of projections on the electrode surface, which makes the electrode surface very rough. Such lithium dendrite causes, together with battery performance decline, separator damages and battery short circuits in severe cases. As a result, a temperature in the battery increases causing a risk of battery explosion and fire.
In order to resolve such problems, researches such as introducing a polymer protective layer or an inorganic solid protective layer to a lithium metal layer, increasing a concentration of a salt of a liquid electrolyte, or using proper additives have been urgently required.