리튬 이온 전지, Lithium-ion batteries

리튬을 주 소재로 해 만들어진 전지로, 요시노 아키라가 발명, 상용화하여 노벨 화학상을 수상했다. 평균전압 3.6~3.8V, 완충전압 4.0~4.35V 이며 스마트폰을 비롯한 휴대기기, 전기자동차 등에 쓰이며 현재 가장 쉽게 접할 수 있는 전지이다. 기존의 NiCd(니켈카드뮴)나 NiMH(니켈 수소) 전지보다 전압과 용량이 커서, "에너지 고밀도" 전지가 필요한 현대 휴대기기에서 적극적으로 사용되고 있다.

공용 규격으로는 CR123의 충전판인 RCR123이나 18650, 21700 등이 있다. 형태(원통형, 각형, 파우치형)에 따라 구분하기도 하고, 전해질, 양극재, 음극재의 재료 등에 따라 구분하기도 한다.

참고로 리튬 이온 전지의 리튬 함량은 낮은 편이다. 2000mAh 용량을 가진 18650 리튬 이온 전지에는 단 0.6g의 리튬만이 들어간다.

리튬 이온 전지의 가격은 점차 하락하고 있는 추세이다. 2020년 평균 kWh당 가격은 $137(약 16만 원)였다. 그러나 이는 제조 원가이며, 소비자에게 판매하려면 보호회로 장착, 안전인증 등 추가 비용이 필요하므로 소매 가격은 이보다 훨씬 비싸다. 특히 2022년에는 여러 악재들로 인해 가격이 반등했다.

특히 리튬은 희토류에 준할 정도로 희귀한 원소이다. 지구의 지각에서 리튬이 차지하는 비율은 고작 0.0017%로 구리보다 적다. 또한 같이 사용되는 니켈과 코발트도 흔치 않다. 때문에 상대적으로 희귀한 리튬 대체하여 바다에서도 쉽게 구할 수 있는 나트륨을 사용하는 나트륨 이온 전지가 상용화되었다. 그러나 이론상 갖출 수 있는 질량 당의 에너지 밀도가 리튬의 절반 정도밖에 되지 않기에 연구개발 및 사용이 활발하지 않다. 그래도 과거부터 이차 전지에서 쓰여왔던 납이나 니켈보다는 가벼우므로 중간 정도의 에너지 밀도를 요구하는 분야(ESS 등)에서 사용될 것으로 예상된다.

참고로 과방전이 되면 내부적으로 비가역적인 손상이 일어나 용량이 크게 감소하게 된다. 방전 종지 전압 미만으로 방전시키면 음극 집전체로 사용되는 구리박이 녹아서 손상되므로 과방전을 방지할 보호 회로와 함께 사용해야 한다. 구리박의 용해로 합선이 일어날 가능성도 있으므로 과방전된 리튬 이온 전지는 안전상 재충전하지 말고 폐기하는 것이 권장된다. 한편 40%~50% 이하인 채로 방치할 때 열화 정도가 커지며 제조 후 오랜 기간 사용하지 않아도 열화가 일어나서 전체적인 용량이 줄어든다.

 

 

 

Akira Yoshino won the Nobel Prize in Chemistry for inventing and commercializing lithium as the main material. It has an average voltage of 3.6-3.8 V and a full-charge voltage of 4.0-4.35 V, and is used in mobile devices and electric vehicles, including smartphones, and is currently the most accessible battery. It is actively used in modern mobile devices that require "energy high-density" batteries because it has a higher voltage and capacity than existing NiCd (nickel cadmium) or NiMH (nickel hydrogen) batteries.

Common standards include RCR123, which is a charging plate of CR123, 18650, 21700. It is classified according to the shape (cylindrical, square, pouch type), or according to the materials of the electrolyte, the anode material, and the cathode material.

For reference, the lithium content of the lithium-ion battery is low. Only 0.6 g of lithium is used in the 18650 lithium-ion battery with a capacity of 2000 mAh.

The price of lithium-ion batteries is gradually decreasing. The average price per kWh in 2020 was $137 (about 160,000 won). However, this is the manufacturing cost, and the retail price is much higher than this because additional costs such as protection circuit installation and safety certification are required to sell to consumers. In 2022, in particular, prices rebounded due to a number of negative factors.

Lithium, in particular, is a rare element comparable to rare earth elements. Lithium accounts for only 0.0017% of the Earth's crust, less than copper. Nickel and cobalt that are also used together are also uncommon. Sodium ion batteries that use sodium, which can be easily obtained in the sea, have been commercialized in place of relatively rare lithium. However, R&D and use are not active because the energy density per mass that can be obtained theoretically is only about half that of lithium. Still, it is lighter than lead or nickel, which has been used in secondary batteries in the past, and is expected to be used in fields that require moderate energy density (ESS, etc.).

For reference, over-discharge causes irreversible damage internally and greatly reduces capacity. If discharged below the discharge termination voltage, the copper foil used as the cathode current collector is melted and damaged, so it should be used with a protection circuit to prevent over-discharge. Since there is a possibility of a short circuit due to the dissolution of the copper foil, it is recommended not to recharge and dispose of the over-discharged lithium ion battery for safety. On the other hand, when left below 40% to 50%, the degree of deterioration increases, and even if not used for a long period of time after manufacturing, deterioration occurs, reducing the overall capacity.