A battery, in concept, can be any device that stores energy for later use. A rock, pushed to the top of a hill, can be considered a kind of battery, since the energy used to push it up the hill (chemical energy, from muscles or combustion engines) is converted and stored as potential kinetic energy at the top of the hill. Later, that energy is released as kinetic and thermal energy when the rock rolls down the hill. Common use of the word, "battery," however, is limited to an electrochemical device that converts chemical energy into electricity, by use of a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes (an anode and a cathode) and an electrolyte solution. A battery is an electrical storage device. Batteries do not make electricity, they store it, just as water tank stores water for future use. As chemicals in the battery change, electrical energy is stored or released. In rechargeable batteries this process can be repeated many times. Batteries are not 100% efficient - some energy is lost as heat and chemical reactions when charging and discharging.
Lithium-ion polymer batteries use liquid Lithium-ion electrochemistry in a matrix of ion conductive polymers that eliminate free electrolyte within the cell. The electrolyte thus plasticizes the polymer, producing a solid electrolyte that is safe and leak resistant. Lithium polymer cells are often called Solid State cells.
Because there's no liquid, the solid polymer cell does not require the heavy protective cases of conventional batteries. The cells can be formed into flat sheets or prismatic (rectangular) packages or they can be made in odd shapes to fit whatever space is available. As a result, manufacturing is simplified and batteries can be packaged in a foil. This provides added cost and weight benefits and design flexibility. Additionally, the absence of free liquid makes Lithium-ion polymer batteries more stable and less vulnerable to problems caused by overcharge, damage or abuse.
Solid electrolyte cells have long storage lives, but low discharge rates.
There are some limitations on the cell construction imposed by the thicker solid electrolyte separator which limits the effective surface area of the electrodes and hence the current carrying capacity of the cell, but at the same time the added volume of electrolyte provides increased energy storage. This makes them ideal for use in high capacity low power applications.
Despite the above comments there are some manufacturers who make cells designated as Lithium polymer which actually contain a liquid or a gel. Such cells are more prone to swelling than genuine solid polymer cells.
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