Sun Lutang used the phrase “yi qi” very frequently in his book, _Taijiquan Xue_, as well as in his Xingyiquan book. He didn’t coin the phrase, of course. For example, the “Taijiquan Jing” used it in the line, “From the feet, to the legs, then to the waist, always there must be complete integration into one qi (wan quan yi qi).” Yang Chengfu quotes this line in his “Ten Essentials.” Sun seems to have an idiomatic take on the phrase, however, whose meaning is not clear to me. He states in _Taijiquan Xue_,
“Taiji is one qi, one qi is taiji. From the standpoint of theory, it is called taiji; from the standpoint of application, it is called one qi (yi ti yan, ze wei taiji; yi yong yan, ze wei yi qi).”
Here, Sun is employing the famous formula, “ti/yong,” first coined by the early thinker Wang Bi (226-249) and revived in the late imperial period which, depending on the context, can mean “theory/application,” “structure/function,” “essence/practical use,” etc. In taijiquan, the ‘ti’ sometimes refers to form practice, and the ‘yong’ to practical applications. Sun Lutang makes reference to many philosophical sources in his writings, including the Yi Jing, the Daodejing, Zhuangzi, Mengzi, etc., and his use of the words wuji and taiji seems to make reference to the neo-Confucian Zhou Dunyi’s vocabulary. To be honest, though, I haven’t studied Sun’s writings closely enough to grasp what he means by this “yi qi / taiji” formulation.